The influence of the number of animals housed together in a cage on antibody production in response to keyhole limpet hemocyanin (KLH) was evaluated among male C57Bl/6 and BALB/c mice. In Experiment 1, male C57Bl/6 mice were housed 1, 6, or 12 animals per cage and primary and secondary responses were evaluated. The secondary, but not the primary, antibody responses of male C57Bl/6 mice were higher among mice housed alone compared to mice housed in groups; differences were observed for both IgM and IgG anti-KLH antibodies. The differential housing effects on secondary IgM antibody responses interacted with the priming dose of KLH. In Experiment 2, the primary and secondary responses to KLH of male C57Bl/6 and BALB/c mice were compared among mice housed alone or 4 per cage. Among both strains, the secondary, not the primary, response was influenced by the number of mice housed in a cage; both secondary IgM and IgG titers were higher among the C57Bl/6 mice housed alone but only secondary IgM titers were higher among the BALB/c mice housed alone. These experiments confirm previous observations that the primary antibody responses of C57Bl/6 mice are not influenced by the number of animals housed in a cage and extend these findings by demonstrating that the secondary antibody responses of C57Bl/6 and BALB/c are influenced by housing condition.