Immunoglobulins A and G were localized by immunoperoxidase labelling in uteri of ovariectomized mice treated with oestradiol-17 beta and progesterone. The administration of oestradiol or progesterone alone to ovariectomized mice for 3 days increased the number of IgA plasma cells from about 1 to 14 per histological section. When the two hormones were administered simultaneously for 3 days the number of plasma cells per section was equal to or greater than with either hormone alone. Treatment with oestradiol followed by progesterone in a sequence that prepares the uterus for implantation resulted in about 31 IgA plasma cells per section. Counts of IgG plasma cells showed similar trends but the numbers were smaller. The results indicate that progesterone increases rather than decreases the number of plasma cells in the mouse uterus. This is consistent with observations on intact mice during oestrus and pregnancy and suggests that the marked increase in endometrial plasma cells at the time of implantation in mice is a response to progesterone acting on an oestrogen-primed uterus.