Laura Z. Hobson, 85, a writer best known for her 1947 novel "Gentleman's Agreement," which dealt with anti-Semitism in America, died of cancer Feb. 28 at a hospital in New York City. She lived in New York.

Mrs. Hobson took four years to write "Gentleman's Agreement," which portrayed anti-Semitism in a time when hotels and clubs were closed to Jews and some jobs and housing were denied to them.

Before writing the book, she told her publisher, Simon & Schuster, "I've got an idea for a book that a magazine will never look at, the movies won't touch and the public won't buy -- but I have to do it."

The story was about a writer, Philip Schuyler Green, who poses as a Jew to learn of anti-Semitism for a magazine article. Among those who came under attack in the story were "liberals" who professed an absence of malice toward Jews, but helped foster prejudice in subtle ways.

Contrary to Mrs. Hobson's stated beliefs, the book immediately won critical and popular acclaim. It first appeared in serial form in Cosmopolitan magazine, and after publication rocketed to the top of best-seller lists, where it stayed for months. Translated into a dozen languages, it eventually sold about two million copies.

The 20th Century-Fox movie version was produced by Darryl F. Zanuck and directed by Elia Kazan, from a screenplay by Moss Hart. Starring Gregory Peck and Dorothy McGuire, it won the Academy Award as the best film of 1947.

Among Mrs. Hobson's other novels were "The Other Father," "The Celebrity," "First Papers" and her last, "Untold Millions," which was published in 1982.

"Consenting Adult," a well-received 1975 novel, explored the relationship between a mother and her homosexual son. Although she declined to say whether or not the book was autobiographical, she said it was true. Her younger son, Christopher Z. Hobson, acknowledged being a homosexual in an essay about the time the novel was published.

Laura Kean Zametkin was born in New York City on June 19, 1900. Her father Michael, a native of Russia, was a labor organizer and editor of the Jewish Daily Forward. Her mother, the former Adella Kean, was a socialist.

Mrs. Hobson later said, "I grew up in an agnostic, broad-minded family. I think of myself as a plain human being who happens to be an American. But so long as there is anti-Semitism in this country, so long as it remains an advantage not to be Jewish, I can never say. 'I am an agnostic,' but must say, 'I am Jewish.'"

After graduating from Cornell University, she became an advertising copywriter. She then spent a year as a reporter with the New York Post before joining the Luce publications in 1934. She did promotion work for Time, Life, and Fortune magazines, and eventually became promotion director of Time before quitting in 1940 to devote herself to writing.

In 1935, her first short story was published, and in 1943, her first novel, "The Trespassers," appeared. It was a story of the disorientation faced by European refugees and their difficulties in reaching and living in the United States. It also was an attack on the country's quota system for admitting immigrants.

In addition to her novels, Mrs. Hobson was the author of hundreds of magazine articles and short stories. Her last book was "Laura Z: A Life," the first volume of her autobiography, which was published in 1983. She was working on the second volume at the time of her death.

Her 1930 marriage to Thayer Hobson ended in divorce in 1935.

In addition to her son Christopher, who lives in New York City, her survivors included an adopted son Michael, also of New York City, and two grandchildren.