John P. Roberts, a promoter of the Woodstock Music and Art Fair in 1969 and a partner in its revivals in the 90's, died on Saturday at Mount Sinai Hospital in Manhattan. He was 56 and lived in Manhattan.

The cause was cancer, said his wife, Rona Roberts.

Mr. Roberts, a businessman who nevertheless pursued fun before profit, and whose taste in music favored Gershwin over the Grateful Dead, was one of the young entrepreneurs who had no experience in the music business when they produced Woodstock, a rock concert that helped define its generation. He was 24 at the time and had recently graduated from the University of Pennsylvania.

With a friend, Joel Rosenman, Mr. Roberts came across the the notion of a large festival concert almost by accident. The men were aspiring writers and had pitched to a television agent the idea of a comedy about two naïve young venture capitalists flush with money but no business ideas. For research, they placed an anonymous advertisement in The Wall Street Journal soliciting ideas for a company made up of ''young men with unlimited capital.'' They received some 5,000 responses.

The partners followed a few leads and wound up going into business with a recording studio in Manhattan, Mediasound. Soon they heard about two other young men, Michael Lang and Artie Kornfeld, who wanted to build a studio in Woodstock, N.Y. The four men discussed promoting a concert with, as Mr. Rosenman put it, ''some of the local talent'' -- including Bob Dylan and the Band -- to raise money for the studio.

The four financed their project with inheritance money from Mr. Roberts and with ticket sales, Mr. Rosenman said.

Taking as their model the Monterey Pop Festival, which drew 28,000 people, the promoters planned for 50,000 fans. The concert, on Aug. 15-17, 1969, drew more than half a million people, not all paying customers, to the site in Bethel, N.Y.

The concert cost $3.1 million to produce and brought in $1.8 million, a deficit that caught Mr. Roberts and his partners unprepared. But they recovered their loss with royalties from film and album spinoffs, and held on to the profitable name and trademark symbol of a dove on the neck of a guitar.

In the years after Woodstock, Mr. Roberts invested in several companies, but avoided the music business. He was also a championship bridge player, and his participation in Woodstock '94 forced the postponement of the Von Zedtwitz Double Knockout Team Championship that year.

Besides his wife, he is survived by children Jennifer and Douglas of New York; brothers William and Keith, also of New York; and a sister, Kathy, of Miami.

Even as a producer of Woodstock '94, Mr. Roberts made it clear that his interests were in maintaining the peaceful legacy of Woodstock rather than in making money, said John Scher, another producer. ''John was a smart businessman,'' Mr. Scher said, ''but he had a lot of heart.''

John P Roberts