The enzymatic and adherence properties of Streptococcus mutans GS5 and S. sanguis ST3, both isolated from human carious lesions, have been compared. During growth in sucrose media, S. mutans GS5 adheres to smooth surfaces approximately three times more effectively than dose S. sanguis ST3. However, strain ST3 does not display sucrose-dependent adherence under nongrowth conditions, whereas strain GS5 displays significant adherence. Although both organisms synthesize both water-soluble and -insoluble glucans, the glucosyltransferases from S. mutans GS5 synthesize much more adherent glucan molecules than do the comparable enzymes from S. sanguis ST3. Both cell types bind exogenous glucosyltransferases synthesized by strain ST3 equally well, whereas cells of strain GS5 bind the comparable enzyme fraction that it synthesizes to a greater degree than do cell of S. sanguis ST3. However, in contrast to the results with cells of S. mutans GS5, the absorption of the glucosyltransferase activity synthesized by S. mutans GS5 to the surface of S. sanguis ST3 results in low levels of subsequent sucrose-dependent adherence. These results are discussed in terms of the molecular basis for the sucrose-dependent adherence of the oral streptococci to smooth surfaces.