The appeal of rhythm and blues has been scrutinized at length by numerous music critics, but the truism routinely expressed by camera-shy teen-agers on American Bandstand still holds true: "It's got a beat and I can dance to it."

At the Wax Museum Saturday night, the beat was there and so were the dancers, as three of the area's top R & B bands demonstrated the lasting appeal of Motown and Memphis soul before a packed house.

Opening were the Soul Crackers, a relatively unknown and surprisingly tight sextet. The combination of Larry Strother's tenor saxophone and Spencer Hoopes' enveloping keyboards projected a vibrantly alive sound, to which Tommy Lepson added gritty and often convincing vocals. Lepson turned "Dancin' in the Streets," for example, into the rousing celebration it was meant to be, and he consistently fared better on the up-tempo tunes in which he didn't have to measure up to the mastery of such soulful balladeers as Percy Sledge.

The Mary Blankemeier Band, in what was billed as its final appearance before several personnel changes, then produced a sassier blend of R & B. Blankemeier was in fine, sultry voice, but the evening really belonged to guitarist Tom Principato, whose stellar guitar work and offhanded, rather Claptonesque singing was particularly enjoyable. Along with saxophonist and flutist Al Williams, Principato nimbly incorporated blues, rock, R & B and jazz elements in his play, creating music that bodes well for the band he plans to form in the near future.

Jr. Cline and the Recliners, a band that has developed quite nicely over the years, concluded the show by drawing largely on the recordings of Wilson Pickett, Al Green and Solomon Burk for inspiration. The band's arrangements seemed cluttered and stagey at times, burdened by unnecessary vamps or, in the case of "In the Midnight Hour," a rather incongruous jazzy trumpet solo.

But these passages were no doubt intended as crowd pleasers (which they were), and frequently were redeemed by the band's brassy momentum and showmanship.