Six hundred male patients were examined for circulating spermagglutinating antibodies by the Kibrick sperm-agglutination test and 20% demonstrated autoantibodies. The high incidence of positive findings may be explained by the select population studied. Of 80 men tested who demonstrated more than 10% agglutination in their semen, but were otherwise normospermic, 41% demonstrated positive titers by the Kibrick method. Of 300 men examined for circulating sperm-immobilizing antibodies by the Isojima sperm-immobilizing test, 6% had autoantibodies. This high incidence of positive findings demonstrates the need for these tests in males who otherwise appear normospermic or demonstrate an unexplained infertility.