The processes of pastnatal development of lymphoid tissues in the mouse were studied. It was found that the weight of the thymus reaches its maximum at 5 weeks of age and decrease gradually thereafter, whereas the maximal weights of the mesenteric lymph nodes and spleen are attained at 4 mothes of age, declining thereafer rather rapidly. The secondary nodules of the Flemming type, having germinal centers, were found to develop during the period between 2 and 6 weeks after birth in the mesenteric lymph nodes, as well as in the white pulp of the spleen. In the splenic red pulp, an extensive erythropoiesis was observed to occur during the peried from bith to the 5th weeks after birth. Such erythropoiesis in the red pulp was found to disappear nearly completely during the period between 6 weeks and 4 months of age and to reappear in the ages older than 5 mothes after birth.