The influence of the estrous cycle on the localization of plasma cell precursors in the genital mucosa was studied in CBA/J female mice by using an adoptive transfer system. Within 24 hr after transfer, [3H]-thymidine-labeled donor mesenteric lymph node (MLN) cells were observed in maximum number in proestral and estral cervices and vaginae (14.4 and 9.7 labeled cells/100 mm2 of tissue, respectively), and approximately 60% of these cells contained IgA and 25% IgG. During metaestrus and diestrus, however, few labeled MLN cells (4.0/100 mm2 of tissue) localized in the genital tract and this reduction affected the IgA-containing plasmacyte population in particular. In contrast, no change was noted in the frequency of labeled cells containing IgA in the small intestine over the course of the estrous cycle, although small but significant changes were noted in the frequency of labeled cells producing IgG. These results indicate that the hormonal changes during the estrous cycle may affect the localization of IgA plasma cell precursors in the genital tract and thus may alter the local mucosal immune response in this site.