Leo Castelli, who became one of the world's most influential art dealers by fostering the careers of such painters as Robert Rauschenberg and Jasper Johns, died Aug. 22 at his home in Manhattan after a short illness. He was 91.

A private service was held that night at his home; Johns was among those there, said Amy Poll, a family spokeswoman.

Castelli's New York galleries were hothouses for major talents in the schools and styles that arose in postwar America and made New York the center of contemporary art. He was in the forefront in promoting the successors to abstract expressionism--pop artists, minimalists and conceptualists. Author Danielle Gardner called him "a modern Medici, the man who sold Europe on contemporary American art."

In addition to Rauschenberg and Johns, Castelli's stable included Frank Stella, Roy Lichtenstein, Claes Oldenburg, James Rosenquist, Robert Morris, Ellsworth Kelly, Kenneth Noland, Cy Twombly, Donald Judd, Dan Flavin, Bruce Nauman, Richard Serra and Andy Warhol, although some defected to other dealers.

Serra once recalled that when Castelli discovered him in 1963, the dealer guaranteed him monthly payments for three years without expectation of selling any of his sculptures.

"It was like getting a Rockefeller grant," Serra said. "Leo has always been generous, supportive, intimate and friendly, a throwback to another century."