Violinist Robert Koff, a music professor here for 25 years who founded the Lydian String Quartet, died last Tuesday in his home in Lexington from a rare blood disease, the Boston Globe reported late last month. He was 86 years old. "Robert's playing was beautiful and effortless, as if he was speaking directly to you," violinist Judith Eissenberg of Brookline told the Boston Globe on Feb. 27. She performs in the Lydian String Quartet Mr. Koff established here in 1979.

Born in Los Angeles on Feb. 12, 1919, Mr. Koff attended the Oberlin Conservatory and received a graduate diploma from Juilliard in 1946. That same year, he teamed up with violinists Robert Mann and Raphael Hillyer and cellist Arthur Winograd at the urging of composer and Juilliard School President William Schuman to create the Juilliard String Quartet.

Mr. Koff, a modern and Baroque instrument specialist, performed with the quartet as its second violinist and helped the group establish itself as the premiere chamber ensemble in the country.

Mr. Koff performed with the group in many of its classic recordings.

During his years in the Juilliard Quartet and on the school's faculty, Mr. Koff also taught at Aspen Music School and Boston University's Tanglewood Institute during the summers.

Mr. Koff left Julliard in 1958 to become a professor and director of performance activities here.

Mr. Koff conducted the University's chamber orchestra, and in the 1960's he became a violinist and the director for the Brandeis Chamber Players. He played in the Brandeis String Quartet for 10 years and was the chairman of the school's music department from 1969 to 1976.

"He was one of the founding fathers of this University, and his presence unquestionably helped to establish the high reputation of the music department," Prof. Marty Boykan (MUS) said, remarking on Mr. Koff's tenure at Brandeis

Mr. Koff also led chamber music programs at Tel Aviv University in the mid-1970's and early 80's. After retiring from Brandeis in 1983, Mr. Koff then worked with musicians at the New England Conservatory and Longy School of Music in Cambridge.

Tom Delbanco, a Lexington neighbor and violin student of Mr. Koff, told the Globe, ''Bob was the ultimate teacher. . . No matter what the level of talent, he'd push, prod, cajole, and stretch students until they outdid themselves. He was always experimenting, whether with different ways to express a musical idea, electronic gadgets, his garden, a bad joke, or a fantasy that he wrote in French."

Mr. Koff's wife, Rosalind Mann, told the Globe, ''One of the things his students loved best about him was that he made education fun."

Besides his wife, Mr. Koff is survived by his sons, Stephen and Jeremy of Los Angeles and Daniel of Sebastopol, Calif.