"What can we musicians say?" asked Isaac Stern, his voice choked with emotion at the memory of a fellow violinist. "How many ways can we say, 'I love you,' to Abe Fortas?" A minute later, on the stage of the Eisenhower Theater yesterday morning, he picked up his violin and, with the aid of four other string players, found a way to express his feelings about the patron of the arts, philanthropist, former Supreme Court justice and amateur chamber musician who had been his friend.

He had the aid of Mozart--the andante movement from the String Quintet in C. "It was his favorite movement," Stern said before joining Margot Collins, Walter Trampler, Barbara Winslow and Albert Feigen in a performance that said it all: that Abe Fortas had played chamber music with these people in his home on Sunday evenings; that he had aided Stern in his epic battle to save Carnegie Hall from destruction; that he had been a vital factor in putting the Kennedy Center, its Terrace Theater and the Hirshhorn Museum on the map of Washington.

"I think it was with us as musicians that he felt most at ease," Stern told a gathering that filled the Eisenhower and included members of Congress and the diplomatic corps, as well as Lady Bird Johnson, Lynda Bird Robb, Chief Justice Warren Burger, Justice Thurgood Marshall and former justice Arthur Goldberg. The tribute to Fortas had been arranged jointly by some of the world's leading musicians and the Kennedy Center, on whose board of trustees Fortas served from 1964 until his death last month.

"We could go to him with our hopes and our problems," said Stern, "and we could always be sure of his appreciation, understanding and support." He spoke eloquently of Fortas' advocacy of the arts ("Without Abe, I doubt that the National Endowments would exist today"), but he returned constantly to Fortas the amateur musician--"amateur" in its root sense of "lover": "How important throughout his life . . . how dear music was to Abe . . . how it put him in touch with man's continual effort to overcome his weaknesses, to find beauty."

Other musicians who joined in the tribute included Mstislav Rostropovich, Eugene Istomin, Leonard Rose, Charles Treger, Jody Gatwood and Judith Serkin. Stern spoke for them all--at least until they turned to their instruments and used the music of Brahms, Rachmaninoff, Mozart and Schubert to say what they felt.

Newsman Eric Sevareid called Fortas a "poor boy from Memphis" who was born "rich in mind, in courage, in that inexplicable instinct for what is tasteful and beautiful and lasting."

"We cannot explain a man like this," he said, "but we can be glad he was around."

Sevareid described Fortas' 1969 resignation from the Supreme Court under allegations of impropriety as "an act of conscience, proving of course that he was exactly the kind of man the court should never be without." He praised Fortas for his defense, often unpaid, of the poor, the young, the falsely accused, the reviled and "those whose good name was their only real asset."

"Absolutely no one did more than Abe Fortas to protect innocent, patriotic Americans from the ravages of that maniacal McCarthy period," Sevareid said. "I, too, am his debtor from that time. There was a period when over 50 percent of the case load in the firm of Arnold, Fortas and Porter consisted of such cases, nearly all of them, one might add, noncompensatory. And when a colleague asked Abe why the firm took them on, he replied, 'Because no one else will.' "

He quoted a letter sent to Fortas' widow, Carol, by Jose Ferrer, who had portrayed him in a film about the history-making Gideon case which upheld the accused's right to counsel: "There are a few people in the world whom one doesn't see often, but it doesn't matter because they are in some way close to you. Abe was one of those."

The hour-long tribute was opened by Kennedy Center Chairman Roger Stevens, who said that he had often depended on Fortas "because of his vast knowledge of governmental affairs as well as his understanding of the arts--and the artists.

"I can think of no better way to pay tribute to Abe than to let the music speak for us," Stevens said.

The music did speak, more eloquently than any words could, as one ensemble after another took its place on the Eisenhower's stage, decked out like a 17th-century French drawing room for the evening's performance of "Tartuffe." Stern was joined by his regular partners, Rose and Istomin, in a movement from the early Brahms trio in B, then followed by Rostropovich and Istomin in the eloquent slow movement of Rachmaninoff's Sonata in G minor. The Mozart andante, coming directly after Stern's remarks, had a special emotional intensity, as did the adagio from Schubert's Quintet in C, which concluded the program.

At the end of each performance, the musicians walked slowly off the stage in a silence broken only by a few coughs--no applause. Stevens had suggested this silence at the beginning of the program, and he was right. One does not applaud what amounts to a kind of prayer, and there are some moments in music for which applause would be inadequate. This occasion was one of them.

"Wasn't the music beautiful?" Carol Agger Fortas said to someone in the family's receiving line after the program. "You don't hear music like that every day." She understated the case. You can hardly hope for an occasion like that once in a lifetime.