Twenty-nine women with serum antisperm antibodies underwent in vitro fertilization and embryo transfer. Half the oocytes from each patient were treated with patient-serum-supplemented medium with antibodies (medium A), and the other half with donor-serum-supplemented medium without antibodies (medium B). In 9 cases the antibodies were against the sperm head and these showed a significant difference in the number of embryos obtained between the two media (p = 0.001): 10.5% of fertilization of type 3 and/or 4 oocytes with medium A and 81.2% with medium B. A lower rate resulted also when the antibodies were against the middle and main portion of the tail, but fertilization did not seem to be affected when they were directed against the tail tip. No definite conclusions are possible, however, since the number of cases studied was too small.