Two old friends and rivals met for the first time in more than 40 years, Saturday night at the Library of Congress.

John Bubbles, who sang the role of Sportin' Life in 1935 for the first production of "Porgy and Bess," was back again singing "It Ain't Necessarily So" and "There's a Boat That's Leavin' Soon for New York." Todd Duncan, the original Porgy, was in the audience, silent until he was lured to the stage and sang a few notes of "Bess, You Is My Woman Now" for the final curtain call.

"I will never forget the thrill of introducing John W. Bubbles to Todd Duncan for the first time in 40 years," Donald L. Leavitt, chief of the library's music division, told the audience in the Coolidge Auditorium.

Bubbles, who began his career as a child performer in 1912, is no longer able to do the tap-dancing that once made him famous. His voice loses some of its resonance after a few minutes, and his memory is not quite perfect. But his performance was memorable and afterward Duncan came on stage to reminisce about Bubbles nearly 50 years ago: "He was such a demon, such a hellion, such a horrible Sportin' Life that we all loved him."

The audience loved him again on Saturday night, when the curtain opened to reveal him, dressed in the costume of a 1930s bon vivant with a plaid suit and gray derby, sitting in an armchair and leaning toward a microphone. "I don't have the choir I had -- but I don't have the voice, either," he told the audience, "so you'll have to help me." Then he sang the solo and led them in the refrains, from "It ain't necessarily so" to "Scuddiwah," taking off his hat as a cue when it was the audience's turn to sing.

At a reception after the concert, Bubbles was approached a bit diffidently by Charles Williams, a Washington tenor who will sing the part of Sportin' Life next February in the Metropolitan Opera's first production of "Porgy and Bess," marking the opera's 50th anniversary. "It was a great moment for me," Williams said, showing a friend the autograph Bubbles had given him. "I feel I know him well because I have listened to his recording so many times, but this was the first time I ever talked to him." Williams has sung the role several times in Europe, in English and German, but never in the United States. He said his interpretation at the Met will be "different" from Bubbles', "more operatic and, I hope, more evil."

The appearance of John Bubbles was the climax of a special program honoring Gershwin, who would have been 86 last Wednesday. Another highlight was the announcement, by Librarian of Congress Daniel Boorstin, that the library plans to open a George and Ira Gershwin Room to house its extensive Gershwin collection when restoration is completed on its 1897 Thomas Jefferson Building. A third was the first American performance of the two-piano arrangement of "An American in Paris," made by Gershwin before he orchestrated it. This version has several minutes of music later omitted from the orchestral score.

"You may have read in The New York Times that the first performance of this work will take place in Carnegie Hall tomorrow evening," Leavitt told the audience. "I want to welcome you to the last out-of-town tryout." The performance, by the French sisters Katia and Marielle Labe que, was fast, energetic, brilliant in its range of dynamics and accents and almost orchestral in its variety of tonal colors. The same qualities could be heard in their exciting performance of Percy Grainger's busy, virtuosic "Fantasy on Porgy and Bess."

But the most perfectly stylized Gershwin piano-playing of the evening came from Dick Hyman, first as a soloist and later as accompanist to Bubbles. As a soloist, he played Gershwin's piano arrangements of "Liza" and "The Man I love" and his own improvisation on "Do, Do, Do." Hyman knows more about vocal phrasing than most pop singers, and he used this knowledge effectively for these pieces, which are really sublimated songs. His dynamics were carefully weighted, his tempos subject to constant, subtle shifts, but the phrasing was the most impressive part of his interpretation. He played as though his piano were a living, breathing thing -- as indeed it was under his fingers.

Four good Broadway-style voices phrased almost as well as Hyman's piano in selections from "Girl Crazy," a 1930 Gershwin spectacular whose hit tunes included "Embraceable You," "But Not for Me" and "I Got Rhythm." These and five other numbers were stylishly performed by Kim Criswell (who has a touch of Ethel Merman in her style) Jeff McCarthy (a fine comic talent) and Jeanne Lehman and Ron Raines, who gave the evening's most convincing dramatic interpretation in "Could You Use Me."