The administration of a single dose of 2.5 mug of microcrystalline estradiol-17 beta from 1 day before and up until 3.5 days after the administration of 3 X 10(5) heat-killed Escherichia coli significantly increased numbers of splenic anti-E. coli antibody-producing cells in male mice sacrificed 4 days after receiving anitgen. Administration early in the proliferative phase of antibody production, i.e., 1 day before or 1 day after the antigen, appeared to increase numbers of antibody-producing cells more than when it was administered at a later time. When given 2 days before the antigen or 2 h before sacrifice no effect was observed. Spleen cells harvested from male animals injected 3 days before with 5 X 10(6) heat-killed E. coli were incubated for 24 h in vitro with estradiol in concentrations ranging from 5 pg to 20 ng/ml. With concentrations of 500 pg to 5,000 pg/ml, significant increases in antibody-producing cells occurred, whereas at concentrations of 20 ng/ml some decrease was observed. The increase in antibody-producing cells was blocked by a mitotic inhibitor. Significant changes in numbers of antibody-producing cells were not observed after a 2-h incubation period. Uptake of titrated thymidine was increased in thymic and spleen cells incubated for 24 h with 500 pg of estradiol per ml; a concentration of 20 ng/ml slightly (but insignificantly) decreased uptake. Findings suggest that estradiol, in concentrations that approximate physiological serum levels in females, enhances mitosis of immunocompetent cells. This phenomenon may have bearing on the better immunological responsiveness of females than males.