Some of the biggest names in documentary filmmaking have denounced a recent agreement between the Smithsonian Institution and Showtime Networks Inc. that they say restricts makers of films and television shows using Smithsonian materials from offering their work to public television or other non-Showtime broadcast outlets.

Ken Burns, whose documentaries "The Civil War" and "Baseball" have become classics of the form, said in an interview yesterday that he believed that such an arrangement would have prohibited him from making some of his recent works, like the musical history "Jazz," available to public television because they relied heavily on Smithsonian collections and curators.

"I find this deal terrifying," Mr. Burns said in a telephone interview from San Francisco, where he is filming interviews for a documentary on the history of the national parks. "It feels like the Smithsonian has essentially optioned America's attic to one company, and to have access to that attic, we would have to be signed off with, and perhaps co-opted by, that entity."

On March 9, Showtime and the Smithsonian announced the creation of Smithsonian Networks, a joint venture to develop television programming. Under the agreement, the joint venture has the right of first refusal to commercial documentaries that rely heavily on Smithsonian collections or staff. Those works would first have to be offered to Smithsonian on Demand, the cable channel that is expected to be the venture's first programming service.