Immune sera to strains of Streptococcus mutans were found to inhibit agglutination of bacterial suspensions to which either high-molecular-weight dextran or sucrose was added. This inhibitory activity was shown to be mediated by antibody of the immunoglobulin G class. A semiquantitative assay was developed which demonstrated cross-inhibition of dextran/sucrose-induced agglutination among several strains of S. mutans. Antiserum to a partially purified glucosyltransferase was found to lack agglutination inhibition activity, consistent with the hypothesis that the dextran-binding antigen detected by the assay is immunologically distinct from the glucan-synthetic enzyme. A model for glucan synthesis and binding consistent with the reported data is described.