The "authentic" instrument movement began with Bach harpsichords and recorders quite a while ago and was steaming through the neighborhood of Schubert with fortepianos and gut-stringed violins when last observed.

Now, suddenly, it has reached Brahms -- a composer who nearly lived into the 20th century -- and we may begin to wonder where it will end. Will Debussy be considered authentic only when played on a turn-of-the-century Pleyel piano? Are purists about to begin insisting that Stravinsky's "Ebony" Concerto sounds right only with a 1940s-vintage clarinet?

Last night, in the Smithsonian's Hall of Musical Instruments -- one of the world's leading shrines of the authentic instrument mystique -- a kind of Brahms performance was heard that has been out of fashion for about half a century. Gut strings were played by cellist Kenneth Slowik and violinist Marilyn McDonald with a minimum of vibrato -- that fuzzing of the sound that string players produce by keeping the left hand in a steady tremble. Constant vibrato is standard technique nowadays -- perhaps because modern metal strings seem to lack warmth without it. But last night the players used it as a sort of accent -- a spice used to heighten moments of drama or pathos, rather than a sort of pancake makeup slathered on the sound.

They also accented with portamento -- the technique of sliding into a note or between notes rather than jumping immediately to dead center. This has been out of style (except for Gypsy music) for about as long as vibrato has been in -- but it is something that Brahms would have expected to hear in his music, as he would have expected the gentler tone of gut strings. It was used rather unobtrusively by Slowik in a beautiful performance of the Cello Sonata in E minor, and rather flamboyantly by McDonald in a brilliant performance of the three most popular Hungarian Dances (Nos. 1, 3 and 5), where it sounded exactly right.

The piano, the Smithsonian's "Paderewski" Steinway, dates from Brahms' lifetime and has been restored to something like its original sound -- less aggressive than the modern instrument. One wonders whether the deepest bass notes should not have a bit more resonance, but its current voicing, in the expert hands of Robert Spano, makes it very apt for chamber music.

The star of the evening was the natural (valveless hunting) horn valiantly played by Lowell Greer, who spent much of the evening with his right hand thrust into the instrument to fulfil functions handled by valves on the modern horn. This instrument was already old-fashioned when Brahms composed his Trio for Piano, Violin and Horn, Op. 40, but he specified its gentle, liquid tone. There were a few small problems of pitch and balance, but the overall effect was exquisite and Greer rightly received the loudest applause of the evening.

The program (to be repeated tonight) is a unique experience: familiar music made unfamiliar in a way that is often a kind of revelation.