Anyone who has ever visited Washington, D.C., knows what a pleasure it is to visit the 16 free museums that make up the nation's Smithsonian Institution. This is where you can inspect the original planes and rockets that circled the skies, learn why African Americans left Southern fields for Northern factories, view artistic masterpieces or gawk at the tiny-waisted dresses worn by former first ladies.

But the Smithsonian is far more than artifacts. Ever since its founding by Congress in 1846, it has faithfully followed its mission to"increase and diffuse" knowledge about our nation. Through exhibits curated by the museum's professional staff, the Smithsonian has taught generations of Americans about their country's social, cultural, technological and artistic heritage.

To be sure, many exhibits have ignited fiery debate over the presentation of our history. But that's what good exhibits should do -- start conversations,

not end them.

Now, the Smithsonian faces a different kind of controversy -- one that threatens to erode its integrity.

For the past year, hundreds of museum curators, professional historians and cultural critics have accused Smithsonian Secretary Lawrence Small of permitting corporate and private donors to purchase the right to define and interpret museum exhibits.

Private money is not the problem. The Smithsonian, which receives two-thirds of its funds from Congress, has always accepted donations from individuals and foundations, who simply expect a heartfelt letter thanking them for their contributions.

Under the direction of Small, a mortgage banker by profession, the Smithsonian has embraced a new kind of commercialism that threatens to turn a public institution into a vehicle for corporate marketing and public relations.

Consider a few of the deals that have drawn heavy criticism from scholars and curators during the past year:

-- In exchange for a $10 million donation, Small offered General Motors the right to name the museum's new hall of transportation. Historians and curators blasted the deal. They worried that GM's sponsorship would result in a sanitized history that ignored its unsavory past -- the fact that it bought up and displaced mass transit systems and produced unsafe and polluting cars for which it has earned a string of criminal convictions. Smithsonian officials insist that GM will not have any influence on presentation."When the public sees the exhibition, it will be clear it is a solely Smithsonian show," Curator Steven Lubar wrote in a letter to The Chronicle last July.

-- In exchange for $28 million from Catherine B. Reynolds, a wealthy philanthropist, the Museum of American History agreed to host an exhibit that honors exceptional individuals. In the original contract, Reynolds had control over the selection of the exhibit's honorees, which included such notables as Martha Stewart and Frederick Smith, the founder of Federal Express. Only after historians protested and extensive negative press lampooned the deal did the Smithsonian turn the decision-making over to a blue-ribbon committee of historians.

-- In exchange for a $7.8 million donation by Fujifilm, the Smithsonian sponsored a"Fujifilm Giant Panda Conservation Habitat." The exhibit shamelessly featured a stuffed panda holding a big Fujifilm sign. Later, the Smithsonian gave Fujifilm its 2001"Corporate Leadership Award," which created the perception that the Smithsonian honor was up for sale.

-- To celebrate Black History Month, the Museum of American History -- in partnership with Kmart Corp. -- assembled a mobile exhibit about African American Sacred Music. Mounted on a 48-foot double expandable trailer with giant Kmart signs emblazoned on each side, the exhibit was to visit Kmart stores, as well as schools. When criticized for using Kmart's name, Small responded,"Why shouldn't they get something out of it? They put up the money for it."

That's the problem. The Smithsonian should not be in the business of advertising corporate logos or products.

Nor should private or corporate money allow donors to influence the nature and content of exhibits. In interviews with The San Francisco Chronicle, for example, museum curators and historians involved in the controversy have revealed how such donations distort the museum's standard procedures. Usually, a professional staff develops ideas for exhibits, for which they draw on recent scholarship. When corporate sponsorship is involved, however, donors not only influence the subject of the exhibit, but sideline other projects in the process.

The Smithsonian Institution is a national treasure that belongs to the American people, not to the marketing departments of giant corporations.

This article first appeared in the San Francisco Chronicle and is reprinted with permission.