Sera from men and women at risk for immunologic causes of infertility were screened for sperm-specific antibodies by a new test devised to assess directly the presence of immunoglobulins bound to the sperm plasma membrane. Passive antibody transfers to antibody-negative sperm of a fertile donor were performed with the use of sera that possessed iso- or auto-antibodies directed against the sperm head, including the acrosome and postacrosomal regions. The ability of these antibody-bound sperm to penetrate the zona-free hamster egg in vitro, a prerequisite for fertilization, was enhanced, as compared with the penetrating ability of antibody-free sperm of the same donor. These results indicate that the sperm-specific antibodies studied in these experiments, although bound to the sperm head, do not cause infertility by interfering with the acrosome reaction or gamete membrane fusion.