LEONARD SLATKIN and the Saint Louis Symphony Orchestra, who are performing at the Kennedy Center this afternoon, originally intended to bring East with them (at least to New York) a new symphony commissioned from Alberto Ginastera. But the Argentine composer's health problems prevented hom fromcompleting the work on schedule, and there is no word as to whether it might be ready next season either.

There is, however, a very welcome "Homage to Alberto Ginastera" in the form of a two-disc set just issued by the Organization of American States in its Inter-American Musical Editions. It contains three major works, all receiving their disc premieres here and all recorded in Washington in the presence of the composer.

Ginastera, of course, has not been a stranger to our city. His operas "Bomarzo" and "Beatrix Cenci" both received their premieres here; the former was recorded in its original 1967 Washington production under Julius Rudel, and the latter was one of the works commissioned for the opening of the Kennedy Center in 1971, when it, too, was conducted by Rudel. In 1978 Ginastera's wife, cellist Aurora Na'tola-Ginastera, introduced the final revised version of his Cello Concerto here with the National Symphony Orchestra under Mstislav Rostropovich, who conducted Ginastera's "Glosses on Themes by Pau Casals" in the same concerts. And, of course, Ginastera's music has figured in the programs of numerous other resident and visiting organizations, as well as the International American Music Festivals, for many years.

The three works in the new set are the Sonata for Cello and Piano, Op. 49, composed in 1979; the String Quartet No. 3 (with soprano), Op. 40, composed in 1973; and the Serenata for cello, baritone and chamber ensemble, Op. 42, also from 1973. All three were recorded in October 1981 by the musicians who presented them live that month in the Kennedy Center's "American Portraits" series, when a more robust Ginastera was on hand for ceremonies and concert, and his wife was one of the performers.

The Quartet, following the example of Schoenberg in his Second String Quartet, includes settings of poems to be sung by a soprano. The texts, in Spanish, are from poems by Frederico Garci'a Lorca, Rafael Alberti and Juan Ramo'n Jime'nez; the performers are Jane Bryden, soprano, and the Washington String Quartet.

The text for the Serenata is from a single source, the "Love Poems" of Pablo Neruda. Carlos Chausson, baritone, and Aurora Na'tola-Ginastera, cellist, are the soloists, with the Theatre Chamber Players under Leon Fleisher.

Na'tola, of course, is heard in the Cello Sonata, too (with pianist Enrique Ricci), a work commissioned by the OAS and dedicated to Na'tola. It is an undisguised musical love letter from the composer to his wife, built in part on the music to which the word "amor" is set in the Third Quartet. The performance is obviously definitive, and those of the other two works may be regarded as hardly less so.

The set comes with a 16-page booklet of annotation, analysis, tributes and photographs, which, however, omits the most essential information on the Quartet and the Serenata, namely the respective texts. Texts are not printed in Spanish or English, nor is there a pre'cis of any sort to give the listener an idea of what is being sung.

An OAS spokesman advised that copyright problems prevented the printing of some of the poems, but that an additional insert being preparedwill include those verses that may be printed and synopses of those that may not. The insert will be included in all future sets and mailed to persons who have already bought the set.

"Homage to Alberto Ginastera" is available by mail from Inter-American Musical Editions, Organization of American States, 1889 F St. NW, Washington, D.C. 20006. The order number is OAS-015, and the price is $12, plus $1.50 for shipping. (The $1.50 covers shipping of an order of any size, by the way, and a catalogue of everything issued in this series so far is available without charge.)