Spleen cell cultures from young adult mice of a variety of strains were stimulated to incorporate tritiated thymidine ([3H]TdR) by a goat anti-mouse IgM antiserum and by purified anti-mu antibodies prepared from this serum. This stimulation was shown to depend upon the anti-mu activity of the antiserum. In addition, ultracentrifuged anti-mu and F(ab')2 fragments of anti-mu were shown to be stimulatory. The anti-mu preparation lacked detectable endotoxin contamination and was also shown to stimulate response by two strains (C57BL/10ScCr and C3H/HeJ) which are unresponsive to the mitogenic effects of endotoxin, while it failed to stimulate a response by cells from a mouse strain (CBA/N) which responds to endotoxin. In addition purified goat anti-mouse gamma, kappa antibodies and rabbit anti-mouse kappa-antib odies stimulated uptake of [3H]TdR by mouse spleen cells, although to a lesser degree than the anti-mu preparation. The cell density, culture requirements, and kinetics of the response are presented.