The spleen of normal dogs (beagles) shows only slight hemopoietic activity, characterized by the presence of megakaryocytes in mitosis and small groups of erythroblasts scattered throughout the red pulp of the organ. Repeated intravenous injection of dextran sulfate, at a dose of 15 mg per kg body weight, produced markedly enhanced erythropoietic and megakaryocytopoietic activity in the splenic red pulp, without concomitant increase in splenic granulopoiesis. The probable existence of a micro-environment adequate for erythro- and megakaryocytopoietic differentiation of stem cells is discussed.