The possibility that sexual behavior triggers neural or endocrine events that influence immune function was explored in male Golden Hamsters (Mesocricetus auratus). Sexually experienced males showed a 62.8 and 64.9% decrease in antibody response to TNP-ovalbumin compared to virgin and sham-mated males, respectively, as measured by ELISA. Antibody deficiency was associated with copulation on the day of immunization and was highly correlated with ejaculatory behavior and the incidence of prolonged intromissions, an indicator of sexual satiety. Although a single sexual encounter was not sufficient to inhibit antibody levels, prolonged intromissions were inversely correlated with titers. Changes in antibody production were not related to plasma levels of cortisol, corticosteroid-binding globulin activity, or testosterone. These results suggest that sexual behavior is immunosuppressive and may play a role in the communicability of sexually transmitted diseases.