ABSTRACT:The responses of spleen cells from male and female BALB/c mice were evaluated to determine if sex‐related variations in immune expression could be found. The immunologic assays used included blastogenic responses to mitogens, and direct and indirect measurement of plaque‐forming cells against particulate antigens. The results indicated that responses of spleen cells from young adult female mice were higher than those of males in all comparative tests. Newborn mice did not demonstrate the sex‐associated immune differences; and among the weanling mice slight differences between male and female spleen cells responsiveness to mitogenic agents were observed.The blastogenic responsiveness of spleens from female BALB/c was greater at proestrus and metestrus, as compared to estrus and diestrus. The peaks of responsiveness corresponded to reported elevated levels of estrogen and pregnenolone during these phases of the cycle. Similar results were obtained with the IgM plaque‐forming cell responses, which were also increased at proestrus and metestrus. This study supports a role of sex hormones in modulation of immune exp