Social interactions can have pronounced effects on reproductive physiology and behavior in a wide range of species. Much less is known about the effects of social interactions on immunity. The goal of the present study was to test the effects of social interactions on both reproductive and immune responses in Siberian hamsters (Phodopus sungorus). Male and female hamsters were housed alone, in same-sex pairs or in mixed-sex pairs for 4 weeks. Animals were then immunized with the antigen keyhole limpet hemocyanin (KLH) and blood samples were drawn 5 days postinoculation. Reproductive tissue masses, testosterone, 17beta-estradiol and cortisol concentrations were measured and immunity was assessed by measuring serum anti-KLH IgM, and mitogen-stimulated splenocyte proliferation. Male hamsters housed with a female had increased testosterone and anti-KLH IgM and elevated splenocyte proliferation compared with males housed alone. Female hamsters housed in same-sex pairs had increased serum IgM compared with females housed with males. Cortisol was elevated in both sexes housed with male conspecifics compared with the other experimental groups. Serum estradiol concentrations did not differ among females in any group. Collectively, the results of the present study suggest that social interactions can alter reproductive responses, but that these changes appear unrelated to changes in immunity. In contrast, the presence of a male conspecific elicits social-stress-induced elevations in serum cortsiol in both males and females, which is generally immunosuppressive. These data support the notion of social-stress-induced suppression of immune humoral and cell-mediated immune responses.