A method has been developed to establish lines of transformed lymphocytes able to produce in vitro the same anti-sperm antibodies as those naturally occurring in immuno-infertile individuals. We utilized lymphocytes from a male donor whose serum contained anti-sperm antibodies of the IgG class up to the dilution 1:10,000, as detected by means of immunobead binding. T lymphocytes were separated from B lymphocytes using magnetic beads coated with anti-T antibody. B lymphocytes were then placed at a concentration of 5 x 10(6)/ml in a 96-well plate, stimulated with phytohaemagglutinin (PHA) and transformed with Epstein-Barr virus. After a few days, only transformed cells continued growing and these were collected. The supernatant was tested for production of anti-sperm antibodies and those transformed lymphocytes shown to be synthesising antibodies directed against the sperm head and the tail were cloned. We obtained a clone of cells producing antibodies of the IgG1 class directed against the head of the spermatozoon. This oligoclonal antibody (F6) recognized a 58-kDa band from a lysate of sperm membranes and was able to reduce the penetration of zona-free hamster oocytes by capacitated spermatozoa.