The simple virtue of human voices in harmony seems irrelevant to today's pop music. Partly because young record buyers demand to be overpowered by electric guitars or drum machines, and partly because, after the Michael Jackson/Madonna Decade, aspiring singers don't want to submerge their egos for the sake of something as old-fashioned as a singing group.

So it's especially gratifying to witness "Spike & Co.: Do It A Cappella," the season opener for PBS's "Great Performances" series (tonight at 9 on Channel 26). The "Spike" is filmmaker-gadfly Spike Lee. The "& Co." is a gathering of six a cappella groups of varying styles. And the one-hour show is a wonderfully conceived appreciation of people singing together, a happy, giddy thing.

Lee neither produced nor directed "Do It A Cappella" (his longtime cinematographer Ernest Dickerson gets the director's credit). He's on hand merely as host and drawing card. But before the program is allowed to hit its groove -- a straightforward presentation of a concert last December at the Brooklyn Academy of Music's Majestic Theater -- we must get through about 15 minutes of waggish preliminaries centered on Lee.

Right off, there are clips of prominent instrumentalists (including Hugh Masekela and Branford Marsalis) feigning offense that the show conceptually excludes them. Then Lee leads actress-director Debbie Allen on a walk through the Brooklyn Masonic Temple, abandoned except for the singing groups they stumble upon in assorted acoustically hospitable areas. This sets up four lip-synced performances (shot music-video style), the tastiest being a basement showdown between the leather-jacketed white quartet Rockapella and the black sextet True Image, adopting the look of a hard-core rap group.

Allen's sole purpose as co-host appears to be making silly small talk with Lee as this prelude unfolds ("Maybe we could do a love scene in your next movie!" she burbles).

When the scene shifts to the Majestic Theater, and the Grammy-winning sextet Take 6 begins "Get Away Jordan," the distractions cease and the music instantly overwhelms. An extraordinarily tight and smooth choir, Take 6 takes its jazzed-up spiritual to a climax of swirling falsettos.

In "The Mining Song," Ladysmith Black Mambazo of South Africa, the show's other star act, emphasizes high-stepping dances, not vocal pyrotechnics. The group gives an exuberant performance, and several members elicit a delighted outburst from the young crowd when they momentarily resurrect that '70s dance the bump.

Two of the show's highlights result from surprising combinations. Rockapella and True Image bring conviction to their team rendition of the Drifters classic "Under the Boardwalk." And Ladysmith Black Mambazo backs up the British female group Mint Julep for a mystic version of "The Lion Sleeps Tonight."

The Persuasions, the only representatives of the glory days of American street-corner harmony, end the show with "Up on the Roof." And yes, it feels as if you're watching an oldies act. But among the brief black-and-white interview segments in "Do It A Cappella" -- which resemble Lee's amusingly interrogatory Levi's commercials -- the reminiscences of the Persuasions are the sweetest. One member says he started as a "singing elevator boy."