The effect of progesterone and prostaglandin F2 alpha on cytotoxic activity of lymphocytes was investigated. Thirty-three pregnant women with a clinical diagnosis of threatened premature labour were treated with the prostaglandin synthesis inhibitor acetylsalicylic acid (2.7 gm/day) for 1 week. Cytotoxic activity of the lymphocytes significantly decreased while their progesterone binding capacity increased by the end of the treatment. Ten healthy pregnant women received a single 15 mg dose of prostaglandin F2 alpha for pregnancy termination. One day after prostaglandin administration lymphocytes showed significantly higher cytotoxic activity and lower progesterone binding capacity than before. Lymphocytes of 44 healthy nonpregnant volunteers were preincubated with prostaglandin F2 alpha and/or progesterone. Prostaglandin F2 alpha (8 nmole) significantly decreased cytotoxic activity of 4 X 10(6) lymphocytes. In the presence of progesterone prostaglandin F2 alpha failed to stimulate lymphocytes.