The effects of panose on glucan synthesis and sucrose-dependent cellular adherence by Streptococcus mutans were investigated. Panose effectively inhibited glucan synthesis from sucrose by glucosyltransferases from S. mutans strain 6715, but increasing amounts of panose increased the release of fructose from sucrose by the enzymes. On the other hand, production of a series of oligosaccharides of increasing size by the enzymes was markedly enhanced in the presence of panose. These results indicate that panose activates the enzymes and that the inhibition of glucan synthesis by panose is due to the transfer of the glucosyl group of sucrose to panose. Sucrose-dependent adherence of cells of various S. mutans strains to a glass surface was also inhibited by panose.