John Van Vleck, 81, a cowinner of the Nobel Prize for physics in 1977 for his work on magnetism and a professor emeritus at Harvard University, died Monday at his home here. The cause of death was not reported.

Prof. Van Vleck was best known for creating the modern theory of magnetism based on quantum mechanics. Much of his research was done in the 1930s, when he published the book, "Electric and Magnetic Susceptibilities."

Prof. Van Vleck was the first to demonstrate the important interaction between the motions of electrons, knowledge which was applied to the development of laser beams and new industrial uses of glass.

"I thought the statute of limitations had run out on me," he said after receiving news of the Nobel Prize three years ago.

The Swedish Academy of Sciences called Prof. Van Vleck "the father of modern maganetism" in awarding him the honor, which he shared with Philip Anderson and Sir Nevill Mott.

Prof. Van Vleck was Hollis professor emeritus of mathematics and natural philosophy. He served from 1951 to 1957 as the first dean of Harvard's division of engineering and applied physics. The 254-year-old Hollis chair is the oldest endowed teaching position in science in North America, a Harvard statement said.

Born March 13, 1899, in Middletown, Conn., Prof. Van Vleck grew up in Wisconsin, where his father was a professor of mathematics.

He earned his master's and doctoral degrees at Harvard, taught there in 1922-23, and then at the universities of Minnesota and Wisconsin. He returned to Harvard in 1934 as an associate professor. He was named professor emeritus on his retirement in 1969.

Prof. Van Vleck is survived by his wife, the former Abigail Pearson of Cambridge.