Investigations have been carried out on the action of several steroid hormones on lymphocyte functions in inbred strains of mice. The recognitive, proliferative and effector phases of allogeneic cell interactions in vitro were assessed using a mixed lymphocyte reaction (MLR) and cell-mediated lympholysis (CML). In MLR containing Balb/c (responder) and C57bl/6 (stimulator) splenocytes DNA synthesis was markedly reduced in the presence of progesterone, cortisol or estradiol. In CML, progesterone and estradiol (1-5 microgram/ml) blocked in vitro generation of cytotoxic lymphocytes, while cultures with cortisol were partially inhibited. None of these hormones suppressed the cytotoxic activity of previously sensitized effector cells generated in vitro. Cultures containing testosterone expressed both normal DNA synthesis in MLR and cytotoxic activity in the CML test. These findings suggest a selective pattern of immunosuppression by sex hormones which may be important in preventing graft rejection or graft-versus-host interactions which may arise as a consequence of fetal engraftment during pregnancy.