The effects of estrogen and progesterone on the spleen of gonadectomized male mice were studied by means of quantitative methods. Estrogen caused an increase in the weight of the spleen. The splenic pulps, red and white, were significantly enlarged; the red pulp had markedly increased in volume. In the red pulp of the control, erythroid cells were most numerous in the various hemopoietic cell lines. Estrogen caused a further increase of erythroid cells, and erythroblasts underwent a 2-fold increase in number. By stereological analysis using EM, erythroblasts could be classified into 3 categories in nuclear and cell sizes: small, medium and large. Large and medium erythroblasts were 3-4 times as numerous in the estrogen-treated group as in the control. The white pulp did not show any histological changes following estrogen injection, and progesterone exerted almost no influence upon the splenic pulps.