The influence of estriol and progesterone on lymphocyte reactivity was examined by testing the cytotoxic effect on human embryonic fibroblast cells of non-pregnant women's lymphocytes incubated with different concentrations of estriol and progesterone and with complete and progesterone-depleted third trimester pregnancy sera. Progesterone, but not estriol, had a significant inhibitory effect on lymphocyte cytotoxicity at concentrations comparable to those present in pregnancy serum. 95% depletion of progesterone from pregnancy sera caused an 80% loss of inhibitory activity on lymphocyte cytotoxicity. These data suggest that the blood level of progesterone in pregnancy is sufficient to depress lymphocyte reactivity and that progesterone is responsible for the greater part of the serum inhibitory activity in at least the later stages of pregnancy.