The mitogenic activity of various purified serotype carbohydrate and cell wall lysate fractions of Streptococcus mutans for murine lymphocytes has been determined. The highest mitogenic responses occurred after 48 hr of stimulation with these carbohydrate preparations when tested on splenic cultures derived from C3H/HeJ and BALB/c mice. The response pattern obtained with optimum concentrations of these preparations was very similar to that seen with the B cell mitogen, LPS. A purified serotype g carbohydrate (M1 g) prepared by mutanolysin enzyme treatment of S. mutans 6715 cell walls stimulated good mitogenic responses in BALB/c nude and nu/+ splenic cultures, suggesting an effect on mouse B cells. This was further substantiated by studies showing that this antigen also induced responses in cultures of B cells purified by nylon wool fractionation, whereas purified T cells did not respond. Mitogenic responses were due to the carbohydrate moiety and not to possible LPS contamination, since alkaline treatment or mixture with polymyxin B greatly reduced responses to LPS; however, no alteration of mitogenic responses to M1 g occurred. Further, LPS nonresponsive C3H/HeJ spleen cells yielded good mitogenic responses to M1 g. On the other hand, periodate treatment, which is known to decompose carbohydrate moieties completely abrogated the mitogenic activity of M1 g antigen. Additional studies with serotypes c and e carbohydrate, which also induced mitogenic responses, suggested that this activity for lymphoid cells may be a common property of S. mutans cell wall carbohydrate.