One of the remarkable things about Peter, Paul and Mary, who are celebrating their 25th anniversary in show business, is how little their music has changed over the years. The trio has made no concessions to contemporary pop technology. As in the 1960's, they continue to make spare acoustic folk-pop, accompanying themselves on two guitars augmented only by a light acoustic bass.
While as soloists their singing is often very raw - Peter Yarrow and Noel Paul Stookey are merely passable folk crooners and Mary Travers's large contralto has an abrasively harsh edge - their singular harmonic blend still weaves magic. It is an almost chemically perfect formula for projecting a genteel solidarity of feeling.
The Minskoff Theater, where they made their Broadway debut on Tuesday evening, was awash with a mixture of nostalgia and camaraderie that grew in intensity during the performance - the first of a six-day engagement. Standing in a semicircle and framed by an exposed brick wall that evoked their beginnings in Greenwich Village folk clubs, the trio turned the vast theater into an extended folk club in which the sell-out audience freely sang along with the trio's hits.
For ''Puff, the Magic Dragon,'' which came early in the evening, they were joined by an 18-member children's chorus. Introducing the tune, which was once controversial for its reported drug references, Mr. Yarrow remarked, ''This song is about the innocence of children and the sadness when they grow up.'' That song and a stirring rendition of Phil Ochs's ''There but for Fortune,'' pointedly addressed to the plight of the homeless, were the high points of the show's first half.
After intermission, each member of the trio took a solo turn, with Mr. Yarrow offering a touching ''Stewball''; Mr. Stookey, a goofy monologue about high technology's sinister incursions into daily life, and Miss Travers singing Lee Holdridge's arrangement of a poem by Edna St. Vincent Millay.
The evening reached a soaring climax with renditions of ''The Great Mandala,'' ''If I Had a Hammer,'' ''Blowin' in the Wind,'' and ''This Land Is Your Land.'' Punching out the lyrics with a vigor and passion that ignored the niceties of perfect harmony, Peter, Paul and Mary achieved an exciting and gratifying sense of communal spirit.