The effect of sperm antibodies derived from the female partner's serum on fertilization and embryo cleavage was evaluated by analyzing the Royal Women's Hospital in vitro fertilization (IVF) data. The results suggest that antispermatozoal isoantibodies detected by the immunobead test (IBT) can interfere with IVF. Thus, in a group of patients with IBT-IgG and IBT-IgA sperm antibody titers of greater than or equal to 10 in serum, a low fertilization rate (15%) was obtained when the wife's serum was used as serum supplement in the IVF culture medium. Where replacement (antibody-negative donor or cord) serum was used in the culture medium, a higher fertilization rate (69%) was obtained (P less than 0.01). These results underline the importance of using replacement serum in cases where the wife has significant sperm antibody levels in her serum. Six pregnancies were obtained in the antibody-positive group (n = 20), five of which occurred in patients with IBT-IgG and IBT-IgA-titers less than 10, for a pregnancy rate of 5/9 in this subgroup. Four of these patients delivered (4/9). Analysis of larger groups of antibody-positive patients is required for further evaluation of these results and ascertainment of the likelihood of occurrence of posttransfer effects of sperm antibodies on the embryo.