2. Hypophysectomy completely inhibits the regeneration of splenic tissue after partial splenectomy. Administration of anterior hypophyseal emulsion restores the regenerative capacity of splenic tissue of the hypophysectomized rat to the normal.

3. The daily administration of anterior hypophyseal emulsion of cattle or of alkaline extracts of fresh or acetone-dried anterior hypophysis during a period of 10 days in normal Bartonellacarrier or Bartonella-free rats results in hypertrophy of the spleen to twice normal size. Normal rats receiving emulsion during a period of 1 month become refractory to the spleen-stimulating effect. The spleen shows little increase in size above the normal at the end of this period. Injections of horse serum or of alkaline extracts of acetone-dried kidney, spleen or liver of cattle did not cause enlargement of the spleen of rats of Barlonella-free stock.

4. The increase in the size of the spleen, following daily administration of emulsion of the anterior hypophysis, is due primarily to a marked hyperplasia of the reticular and endothelial cells of the red pulp. The follicles also increase in size. Clusters of reticular cells containing numerous lymphoid elements appear throughout the splenic pulp. The reticular tissue of the bone marrow is similarly increased. There is a striking increase in the number of hemocytoblasts and megakaryocytes. The Kupffer cells are not affected.

5. The spleen-stimulating factor is not present in the acid extract of anterior hypophysis that contains thyrotropic and adrenotropic factors. It is present in some degree in an alkaline extract of fresh anterior hypophysis containing growth and gonadotropic factors. It is also present in alkaline extracts of acetone-dried anterior hypophysis relatively free from growth hormone.

6. The presence of a spleen-stimulating factor in the anterior hypophysis is suggested by these experiments.