ABSTRACT: The effect of progesterone and prostglandin F2α 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 F2α 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 F2α and/or progesterone. Prostaglandin F2α (8 nmole) significantly decreased cytotoxic activity of 4 × 106 lymphocytes. In the presence of progesterone prostaglandin F2α failed to stimulate lymphocytes.