MENDELSSOHN, one of the most beloved composers on the strength of a handful of works, continues to be one of the most underrated as well, as we are reminded most pointedly by the apperance of a new Deutsche Grammophon four-disc set in which all his string quartets are performed by the Melos Quartet of Stuttgart (2740.267).

It will come as a surprise to many who consider themselves knowledgeable that Mendelssohn wrote enough quartets to fill eight sides. The Op. 12 Quartet in E-flat is well-known because of the endearing Canzonetta, and Op. 44 No. 1 in D major is performed frequently. The other two Op. 44 quartets turn up now and then, and a little less frequently we encounter Op. 13 in A minor, whose Intermezzo is as characteristic of Mendelssohn as the Op. 12 Canzonetta. But there are three additional works that are far less likely to be heard in a "live" context.

There is, first of all, a Sixth Quartet, Op. 80 in F minor, a dramatic and uncharacteristically dark-hued work composed a few months after the death of Mendelssohn's beloved sister, Fanny, in May 1847, the last year of his own life. Because this music does not conform to the stereotype of the "elfin" spirit that is indeed present in Opp. 12 and 13, it is considered somehow less worthy of performance; in any event, we hardly ever have a chance to hear it.

Then, from the other end of the sequence, there is a marvelously attractive Quartet in E-flat composed in 1823 by a brilliant 14-year-old. Why Mendelssohn chose not to publish this work is a mystery, for its quality places it clearly apart from such juvenilia as the double concertos and even the string symphonies, and in the same class as the substantial Symphony in C minor composed the next year (one year before the magnificent String Octet).

Finally, there are four pieces collected posthumously under the heading Op. 81: an Andante and Scherzo composed in 1847 and a Capriccio and Fugue produced about 10 years earlier. These are sometimes presented in a different sequence, and that probably makes little difference, though I think the one indicated here makes the most sense.

Mendelssohn's craftsmanship has never been in question, but many listeners who thought they knew this composer will find the level of inspiration maintained throughout the quartets--composed between the age of 14 and the year he died at age 38--downright astonishing. They will surely find these performances surpassingly fine. There have been some outstanding recordings of some of the individual quartets in the past, but none, I think, quite as persuasive as these. The Stuttgarters understand this music down to the ground, and their realizations of the earliest and latest of the quartets are, in the truest sense, revelatory.

Some or all of these discs may be issued separately later on, but don't wait for that; get the set now and start enjoying it. The recording itself is superb, and the documentation tells us just what we have to know about the music.

The same label also has issued a three-disc set of all four of the string quartets of Alexander Zemlinsky and the Quartet No. 1 of Hans Erich Apostel, played by the La Salle Quartet (2741.016). The La Salle has been performing Zemlinsky's quartets in recent years, and DG issued No. 2 on its own a year or two ago (2530.982); the other four sides here are new and digitally recorded.

These are fascinating works. The First Quartet, composed in 1896, shows certain likenesses to the style we associate with Dvora'k; the later ones--composed in 1913, 1924 and 1936--reflect Zemlinsky's admiration of Mahler and also remind us that he was the teacher of Arnold Schoenberg, in particular the Schoenberg who composed the early quartets and "Transfigured Night."

Apostel, whose name must be unknown to most American music lovers, was a pupil of both Schoenberg and Berg; he died in Vienna in 1972, at the age of 71. The idiom of his Op. 7 Quartet is similar to that of Zemlinsky, but no less individual. Provocative music, in the best sense, and certainly well served here: It is pertinent to note that Apostel composed his Second Quartet in 1956 for the La Salle, and one hopes it, too, will materialize in recorded form before long.

From A to Z, or from Z to A, this is a stunning release. The documentation includes a three-page chronology relating events in Zemlinsky's life to happenings in the arts, politics, science etc., and a personal reminiscence of Apostel by Walter Levin, the La Salle's leader.