Kansas City, Mo. ? The Nelson-Atkins Museum of Art has obtained one of the country’s largest collections of American photography, an estimated $65 million treasure featuring works by more than 900 artists, the museum planned to announce today.

The Hallmark Photographic Collection, a roughly 6,500-piece assortment of works owned by Hallmark Cards Inc., will be permanently turned over to the Kansas City museum, officials at the company and Nelson-Atkins said.

“It’s one of the finest holdings of American photography ever put together,” said Keith Davis, the longtime director of Hallmark’s fine arts program who now also will be curator of photography at Nelson-Atkins. “It’s a collection that would be exceedingly hard to duplicate today under any circumstances.”

The contents of the Hallmark Collection are widely varied – from 320 works by influential photographer Harry Callahan, believed to be the largest such holding in the world, to images by Alfred Stieglitz, whose images helped photography become recognized as an art.

For a lover of photography it is considered a jewel. There are iconic Life magazine photos and William Wegman’s outrageous pictures of dogs taking on human roles. There are works by American icon Andy Warhol, renowned celebrity photographer Annie Leibovitz the legendary Dorothea Lange – the list goes on and on.

A formal price tag has not been put on the value of the collection, but it has been estimated near $65 million.

Neither Hallmark nor the museum were making public the terms of their agreement. Representatives said only that a “significant portion” of the collection was donated and the balance was purchased with funds from the Greater Kansas City Community Foundation.

For Nelson-Atkins, it fills a void. The museum is known for its Asian art, European paintings and modern sculpture, but years ago closed its space devoted to photography and other works on paper.

“This adds another component and a distinctly modern and approachable component to the collection,” said Scott Stuart, a spokesman for the museum.

The collection will be housed in the museum’s new $350 million Bloch Building, to open next year, and will rotate several times annually, Stuart said. Thirty pieces go on display beginning today.

Kansas City-based Hallmark has been collecting fine art since 1949, and its photography collection was started in 1964.