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Works by Takashi Murakami on display at the Broad museum in Los Angeles, which opens on Sept. 20.Credit Monica Almeida/The New York Times

LOS ANGELES — Traditional art museums are some of the most conservative and controlling institutions on earth. They are built as vaults to preserve the past, and as monuments to edited histories. In the Gilded Age America of a century or so ago, many new museums were also monuments to private collectors — Henry Clay Frick, J. P. Morgan, Isabella Stewart Gardner — who strove to shape and fix an image that history would have of them, as enlightened power brokers of their day and benefactors to the future.

In our present Gilded Age, private collection museums are again proliferating, but with a difference. Most are devoted to new art, art without a past. The stories they tell are not yet history, but exist in a state of flux. The very definition of collecting, in a time of speculative buying, is now up for grabs. Shouldn’t these changes radically alter the old museum model, loosen it up, make it more experimental, shift its identity from locked treasure house to clearinghouse for fresh ideas?

These questions arise as one of the most eagerly anticipated private museums of contemporary art in the country approaches its opening here next week.

Called The Broad (pronounced brode) and housed in a $140 million, three-story building by Diller, Scofidio and Renfro, it enshrines the collection of some 2,000 works owned by Eli and Edythe Broad, two of this city’s leading philanthropists.

Mr. Broad, a billionaire who made his fortune in home building, has arguably had more impact shaping this city’s cultural identity than anyone else in recent times. For nearly 50 years, he and his wife have been among the country’s most assiduous contemporary collectors. They began picking up work by hot young artists — Jean-Michel Basquiat, Cindy Sherman — in Manhattan in the early 1980s, later filling in historical blanks and doing some buying in their own California backyard.

The inaugural display is clearly intended to show the collection in representative form, and does. The museum’s founding director and chief curator, Joanne Heyler, has installed some 200 works more or less chronologically on the building’s skylighted third floor, beginning with a clutch of classic pieces by Jasper Johns, Robert Rauschenberg and Cy Twombly. Mr. Johns’s 1964 “Watchman” is a star; a blood-red Rauschenberg abstraction from a decade earlier is less familiar, but the Broads cashed in a Van Gogh drawing to acquire it.

Andy Warhol, whose Campbell’s Soup Can pictures Ms. Broad first saw (but didn’t buy) as early as the 1960s, has a small gallery of his own; Roy Lichtenstein has a larger one. He is a Broad favorite; they own 34 pieces (there are 10 here), as is his successor in formally polished Pop, Jeff Koons, of whose works the Broads have the greatest number in private hands. Is this something to brag about? An argument can be made that Mr. Koons’s work usefully casts a cold eye on an American, and now global, addiction to bright, empty, throwaway things. But what happens when a presumably critical art is indistinguishable from its target, or is not critical after all? Then chances are good it’s headed for history’s scrap heap, eventually if not now.

Speaking of critical commentary, in an inspired compare-and-contrast move, Ms. Heyler has inserted a 1995 panoramic city painting by the Los Angeles artist Lari Pittman into the Koons gallery. Mr. Pittman’s work, too, comes out of a Pop corner and is formally airtight. It’s also conceptually razor-sharp. It deals with all the American subjects Mr. Koons does — sex, religion, celebrity, death — but with a focus and bite that he lacks.

The concentration of Los Angeles art is the most interesting aspect of the inaugural show, at least for this East Coast viewer. Ed Ruscha’s laconically meticulous word paintings and John Baldessari’s recycled film images may fit the collection’s clean-lined Pop proclivities, while the acidic zaniness of Mike Kelley’s work does not, but the Broads bought plenty of it over the years. I’m always glad to see it, and I’m even gladder to encounter things I’ve never seen, like the sculpture called “Bateau de Guerre” by the apocalypse-minded Chris Burden, who died in May. A whirring, blinking death star made of gas cans and toy guns, it wasn’t in the recent Burden retrospective that came to New York.

I wish there were more things like it here, under-known, offbeat, less than neat. And there could be. With a reported $200- million-plus endowment and additional funds for acquisitions — nearly that of the Los Angeles County Museum of Art and the Museum of Contemporary Art combined — the Broad will be doing a lot more buying. And it would be good if this museum started to stray from the blue-chip-masterpiece path that winds its way from Mr. Koons on the third floor to a gallery on the first floor of big, bland, abstract pictures by Mark Grotjahn and Christopher Wool, artists who, because they cover walls with work that is indisputably “art,” have become universal collection staples.

Their presence here makes the Broad feel ordinary, old-school, predictable. A tight, unadventurous building design doesn’t help. The exterior, with its sheets of perforated, biomorphic white cladding — the color and texture of gefilte fish — is eye-filling but unmagical, though there are nice touches inside. The cavernlike lobby sets up a mood of mystery. The third floor skylights are a pleasure, as are occasional breaks in the white-box gallery walls that give glimpses onto the street.

The street is Grand Avenue, which Mr. Broad, in consultation with the city government, has long planned to develop into a downtown cultural district. The Broad is part of that plan. So is the Frank Gehry-designed Walt Disney Concert Hall next door to it, and the Museum of Contemporary Art, which Mr. Broad helped found and has generously supported, directly across the street. In a stretched-out, traffic-clogged city it takes a long time to travel anywhere. You need a good reason to go where you’re going. By offering free admission, Mr. Broad intends his museum to be a popular destination.

It surely will be while it’s new, and in the news, and could continue to be. The Broads have always viewed their holdings as a public asset that they make accessible through an active institutional loan program. They refer to their holdings as a lending library, with items regularly leaving for other museums and returning. This traffic flow, enhanced by the arrival of new acquisitions, should encourage people to make repeat visits, knowing they are likely to see new things each time.

But even with this mechanism for flexibility, the Broad is a museum of an old-fashioned kind. It’s been built to preserve a private collection conceived on a masterpiece ideal and consisting almost entirely of distinctive objects: paintings and sculptures; precious things. Apart from most of the objects being new, or at least not old, the Broad could have existed, pretty much as is, a century ago.

But, of course, art itself has changed. It is no longer only about things, hasn’t been for decades. Since the great surge of dematerialization introduced by conceptualism in the 1960s, art has been about, among other things, ideas, actions, sounds, performance, networks, communication. The Broad will have to catch up with this alternative history, a history that the audience it wants to attract and hold already knows. What better way to do so than through collaboration with an institution that has a stake in exploring the same history, meaning, of course, the Museum of Contemporary Art across the street.

The two could share, to their mutual benefit, space, expertise and personnel. What they already share is a tough time for museums and a history with Mr. Broad, who, over a tireless half-century, has done wonders for art in this city, and, with the opening of his museum, is about to do more.