Over the last decade, numerous jazz musicians have been experimenting with groups that focus on a specific family of instruments -- the equivalent of classical music's string quartets and brass ensembles. It's not a new concept in jazz -- there have been many precedents, including such recent examples as the commercially successful band Supersax and Max Roach's percussion ensemble M'Boom -- but interest in the form continues to grow.

At the forefront of the movement is the World Saxophone Quartet, more of a cooperative than a band, really, since each member composes for the group and is a band leader in his own right. The group's latest album, "Dances and Ballads" (Elektra -- Nonesuch 9 -- 79164 -- 4), isn't its best; it lacks the sheer exhilaration of its live recordings and overall thoughtfulness and consistency of its recent Duke Ellington homage. Yet "Dances and Ballads" is very good, and, as always, derives much of its power and beauty from the distinct personalities within the group.

Where the album falls short is in its repetition of familiar motifs. The opening number "Sweet D," for example, recalls several of the band's previous recordings, as Hamiet Bluiett's percolating baritone sax underpins the squabbling horns of Julius Hemphill, Oliver Lake and David Murray. For all its swagger and vitality, the now traditional reprise of the band's theme, "Hattie Wall," is also beginning to wear thin.

Far more refreshing is Murray's "For Lester," a haunting film noir-like tribute to saxophone giant Lester Young, softly shaded by the wistful, evocative colors of flute and alto clarinet. Bluiett's "Full, Deep and Mellow" is similarly inspired by sounds of jazz past, but here the mood is rich and Ellingtonian. Another delight, albeit in a much brighter context, is Lake's "West African Snap." It's high-life music sung by twin soprano saxes and set to an infectious dance beat.

The Clarinet Summit

Ellington's legacy also inspires the work of the Clarinet Summit, which isn't all that surprising since its senior member Jimmy Hamilton is an Ellington alumnus. The rest of the band is made up of WSQ holdover David Murray, John Carter and Alvin Batiste. While they all play B-flat clarinets, except for Murray, who uses a bass clarinet, the band's first studio album, "Southern Bells" (Black Saint BSR 0170), is full of vibrant colors and lush textures, with the two Ellington tunes, "Don't Get Around Much Anymore" and "Perdido," tempering the more raucous performances.

The members not only span three generations of jazz, they represent different regional influences as well. The most gregarious is New Orleans native Batiste, whose own tunes, "Fluffy's Blues" and "Beat Box," are alternately explosive and playful. But the ensemble performances are what really shine here, with the best of them being "Mbizo," Murray's passionate tribute to the late South African jazz musician Johnny Dyani.

First Brass

Another band experimenting with a similar idea is First Brass, an enterprising European brass quartet. The most impressive thing about the group's album "First Brass" (M-A Music NU 1580) is the high level of musicianship and arranging skills. Superb technicians all, the members of First Brass apparently set out to see if the sound of a modern band could be simulated by an ensemble using only brass instruments, in this case trumpets, fluegelhorns, trombones, euphoniums and tuba. On that level at least, they succeeded brilliantly, duplicating everything from the percussive pop of an acoustic bass to the electronic wheeze of a synthesizer.

Still, there's something almost clinical about this recording, which is so tightly arranged there's little room for personal expression. Indeed, performances like "Don't Shoot the Banjo Players," in which the brass imitates a strumming banjo with remarkable accuracy, seem intended more to show off the resourcefulness of the players and the arranger than anything else. If you didn't know that brass players were involved -- or worse, didn't care much -- you probably wouldn't pay much attention to this little bagatelle or to the others that make up most of this album. While there are exceptions, notably the sensuous "The Lady in Blue," they are few and far between.

Repercussion Unit

Unlike these other groups, the six-man Repercussion Unit had to plug in a couple of instruments before recording its album "In Need Again" (CMP 31ST). Bass synthesizers provide an undulating rhythm on several tracks, but the principal focus is acoustic, with the band employing all manner of drums, gongs, bells, wood blocks and even the occasional frying pan.

Unfortunately, the band's interests may be too eclectic for its own good. The album embraces everything with a big bear hug. There's new-age noodling and Frank Zappaesque burlesques and breezy Caribbean-flavored tunes and tribal dances and street funk and, yes, even touches of jazz, though only touches. Spread over two albums there's enough rhythmic power, irreverence, imagination and oddities here to keep just anyone listening for part of the time. It's the other part that will prove trying.