Seventeen Rh negative pregnant women who had Rhesus alloantibodies were investigated serologically. The antibody-mediated cellular cytotoxicity (ADCC) against target Rhesus positive human erythrocytes was determined. The investigation was repeated in the first, second and third trimester of pregnancy and after delivery. There was no correlation between ADCC and antibody titre. The antibody titre and the cytotoxic index increased during pregnancy, but the cytotoxic index decreased after delivery. The cytotoxic index showed individual characteristics and was quite independent of the titre of anti-D. There was no correlation between the heavy chain type of anti-D and the cytotoxic index. Our data suggest that the individual variations during pregnancy of the supposed cytotoxic antibody acting together with anti-D play an important role in pregnancy and delivery.