The primary immune response patterns of young adult male and female hamsters injected with SRBC cells were studied and compared. The females were shown to produce higher numbers of PFC per spleen than did males. Similar results were obtained using aged males and females. These results were found to be independent of circadian rhythm for the males tested. Effects of pre- and post-pubertal gonadectomy on the primary immune response suggest that female advantage in terms of specific immunoglobulin synthesis depends primarily on the relative absence of male gonadal hormones.