The program called them sonatas for violoncello and piano: Beethoven's Third in A and the two by Brahms in E minor and F major. But last night in the Kennedy Center Concert Hall, they were sonatas for Rostropovich and Serkin. They could hardly have been in better hands.

The two performers have set the standard for their respective generations in a certain kind of music-making: Rostropovich as the impassioned, technically brilliant virtuoso of the cello, which has become a virtuoso instrument with a mass following only in the present century; Serkin as the modern master of form, balance and restrained, luminous intelligence in a keyboard tradition that dates back to the time of Bach.

Such, at least, are the images. Neither impression is completely accurate; Serkin's technique is flawless and can be dazzling when he chooses, while Rostropovich masks a razor-sharp intelligence under his teddy-bear persona, and his sense of overall structure and fine nuance in his chosen repertoire is second to none. The interest of last night's performance in the Kennedy Center Concert Hall--the first time they have played together though they have been friends for years--was as much in the encounter of two strong, firmly defined and strikingly different personalities as in the music they had chosen to play. As for the three sonatas, opinions may differ on which of the three is the best, but they are by a considerable margin the three greatest works composed for this pair of instruments.

This year, tragedy has been the occasion for great chamber music in Washington. Not long ago, it was a farewell tribute to the late Abe Fortas that brought some of the world's great chamber musicians together. Last night, it was the destruction of the Filene Center at Wolf Trap that put Rostropovich and Serkin together on the same stage in a benefit concert that raised an estimated $125,000 for the rebuilding fund. Wolf Trap founder Catherine Filene Shouse was at the concert, sitting in the presidential box--her first public appearance since the accident that hospitalized her.

Introducing the concert, Robert Keith Gray, chairman of the Wolf Trap Foundation, hailed her as a "very tough but very gentle lady" and quoted her statement on why she had given the performing arts center to the nation: "I was living out at Wolf Trap Farm and it was such a bother to have to go into Washington when I wanted to be entertained."

Last night, although she did not feel able to attend any of the embassy parties that preceded the concert or the champagne reception that followed it, she was royally entertained by a performance that was very tough, very gentle and in touch with every nuance of this superb music.

The program opened with the Beethoven, a prime product of his middle period that seemed to pass beyond its normal limitations in this performance. The first impression was one of unusual subtlety in music that is essentially straightforward and lyric: the fantastic gradations of tone that Rostropovich has at his command, so that each phrase can be spoken in a distinctive voice. While Rostropovich used a rainbow of tonal colors, Serkin concentrated largely on line, shading his performance in subtle grays. And despite what may sound like a conflict of styles, they worked together splendidly: The chamber music process is a dialogue of equals, and total unanimity (so desirable in orchestral playing) would rob it of essential flavor.

The two Brahms pieces go well together on a program, the gently lyric and sometimes playful No. 1 contrasting neatly with the stormy No. 2. This music is at the center of Rostropovich's repertoire, programmed into his fingers, through years of familiarity, in a way that can hardly be the case for Serkin. On the other hand, Serkin knows the soul of Brahms as it is known to no other pianist, and that knowledge permeates his playing even in music that is on the periphery of his regular repertoire.

For a few bars at the beginning of No. 1, it looked like Rostropovich might become a virtuoso with accompanist--a phenomenon still sometimes mistaken for chamber music. But there was no danger of this with Serkin at the keyboard. Once or twice, Rostropovich even seemed to be the restraining influence, more deliberate in his phrasing and softer in the molding of melodic contours. At times, these differences of nuance seemed to be the instruments themselves discussing the music. The program listing was right, in a sense. In these hands, the sonatas were a dialogue not for cello and piano but for the ultimate Cello and Piano.

Serkin and Rostropovich are planning to record this material for Deutsche Grammophon, and the result will certainly be memorable, whether or not it duplicates what happened last night.