Female Fischer rats have a significantly (P less than 0.01) lower response to mitogens (phytohaemagglutinin and pokeweed mitogen) in whole blood cultures, than do male. In vitro cultures were carried out in 96-well micro-test plates with 5% blood over a period of 64 h, with a pulse of [3H]thymidine at 48 h. Ovariectomy resulted in a significant (P less than 0.01) increase in mitogen response, raising it to the level of normal males. Orchidectomy caused a small, but consistent rise in response. Injection of oestradiol valerate (at 0.5 mg/week, to yield physiological levels) suppressed the response of gonadectomized animals of both sexes to the level of normal female rats. Testosterone oenanthate had little effect unless administered in conjunction with oestradiol valerate, when a much greater depression of response was noted. Suppressor cells, or suppressive serum factors, could not be detected in the blood of normal females or oestrogen-treated animals during in vitro mixing experiments. Although it required 3 weeks for the ovariectomized animals' response to rise to the level of normal males, only 3 days of oestradiol valerate treatment was necessary to suppress the response again. Whereas it seems clear that oestradiol is responsible for the suppressed response of female Fischer rats, its mechanism of action has not been revealed by our experiments.