A crude glucosyltransferase (GTase) preparation was obtained from the culture supernatant of Streptococcus mutans strain MT703 (serotype e) by 50% ammonium sulphate precipitation. Antiserum specific against the GTase was prepared by immunizing rabbits intramuscularly with the GTase in Freund incomplete adjuvant, followed by GTase without adjuvant intravenously. Gamma globulin fractions of the antiserum and normal serum were partially purified by 1/3 saturated ammonium sulphate precipitation. The antibody strongly inhibited the GTase activity (greater than 90%) of type c, e and f S. mutans, whereas the GTase of type a, d and g was not affected by the antibody. The GTase from type b S. mutans was slightly inhibited. The adherence of viable cells of type c, e, and f S. mutans to a glass surface due to synthesis of glucan by the cell-associated GTase was also significantly inhibited by the antibody to the enzyme. These results suggest that type c, e, and f and types a, d, and g S. mutans can be separated into two major groups in terms of the immunological relationship of GTase.