Semen samples from infertile men were assessed for sperm autoimmunity by direct immunobead assay for immunoglobulin (Ig)A and IgG sperm antibodies and mucus penetration test. Immunosuppressive activity in seminal plasma was measured by an in-vitro bioassay employing dose-dependent inhibition of phytohaemagglutinin-induced activation of rat thymocytes, in the presence or absence of hydroxylamine (0.1 mM), an inhibitor of polyamine oxidation. All seminal plasma samples, regardless of autoimmune status, caused inhibition of T-lymphocyte activation, and hydroxylamine reduced this bioactivity by appproximately 50%. Dialysis (<3500 molecular weight) also significantly reduced seminal plasma bioactivity, both in the presence and absence of hydroxylamine. In the presence of hydroxylamine, there was a negative correlation between IgA, but not IgG, antibody concentrations and lymphosuppressive activity in seminal plasma. Antibody-positive samples displaying impaired sperm function, as indicated by the mucus penetration test, had reduced activity compared with other samples. In contrast, there was no relationship between sperm autoimmunity and lymphosuppressive activity assayed in the absence of hydroxylamine. The data indicate that T-lymphocyte inhibition by human seminal plasma is due to multiple factors, and reduced amounts of these factors may contribute to the development and/or persistence of sperm autoimmunity in infertile men; however, differences in polyamine substrates available for oxidation in semen do not appear to be a major contributing factor.