Antisperm antibodies appear to impair reproduction. Follicular fluids, ascites and sera taken at the time of oocyte recovery from patients undergoing gamete intrafallopian transfer (GIFT) were examined for the presence of sperm immobilizing antibodies and their immunoglobulin classes were determined. Three out of ten patients were found to have sperm immobilizing antibodies in their sera. The immunoglobulin classes of the antibodies were IgG and/or IgM, and no IgA class antibodies were detected. The same classes of antibodies found in sera also existed in follicular fluids and ascites in these patients. Furthermore, titers of the antibodies in follicular fluids and ascites were consistent with those of patients' sera.