The lectin concanavalin A (Con A) agglutinated the cells of 13 of 15 strains of the seven serotypes of Streptococcus mutans in an 18-h incubation period. Strains of types a, d, f, and g agglutinated within 2 h. Strains of a, d, and f were also agglutinated in 2 h by the castor bean lectin RCA. S. sanguis, S. salivarius, S. bovis, Actinomyces viscosus, A. naeslundii, and Lactobacillus plantarum were agglutinated within 2 h. The S. mutans type f polysaccharide was precipitated by Con A. The a, b, c, d, and e polysaccharides were not precipitated. Glucan from d and e strains of S. mutans and dextran T2000 were also precipitated by Con A. D-glucose inhibited the agglutination of type f cells by Con A and the agglutination of type d cells by D-galactose. The quantity of [acetyl-3H]Con A bound was not proportional to the degree of agglutination. Cells grown in sucrose medium bound more Con A than those grown in glucose medium. After treatment with dextranase, the sucrose-grown cells bound twoto fourfold more Con A. The binding of Con A to the type-specific polysaccharide or to teichoic acid could not be determined by the use of specific antibody due to the binding of Con A to the antibody globulin on the cell surface. Con A bound to S. mutans cells did not inhibit the activity of cell-bound glucosyltransferase, glucan synthesis, and in vitro adherence. Bound Con A also did not inhibit the ability of heattreated cells to bind glucosyltransferase, synthesize glucan, and produce in vitro adherence. Streptococcus mutans, an organism that is associated with the development of dental caries in man and animals (43), has been classified into seven serotypes (a tof) (7,41). These type-specific antigens are polysaccharides located in the cell wall (15, 17, 22, 28, 30, 34, 35, 52). Recently plant lectins (agglutinins) have been found to react specifically with sugar residues at the nonreducing terminals of polysaccharides and glycoproteins (12, 31, 32, 45), frequently resulting in agglutination of bacterial cells (25, 26, 32, 44, 50). Among these lectins, concanavalin A (Con A) isolated from jack beans has been shown to react with polysaccharides containing aD -glycopyranosyl, aD -mannopyranosyl, or ,8-D-fructopyranosyl residues (13). On the other hand, an agglutinin (RCA) isolated from castor beans (Ricinus communis) was found to be specifically reactive with polysaccharide or glycoproteins containing a and /3-D-galactose residues (39, 40, 49). The reaction of Con A with teichoic acids has been reported by a number of workers (2, 5,8, 11, 24, 42).