Erythremia (polycythemia vera) is generally considered a primary disease of the hematopoietic system, of unknown etiology. Despite the great increase in the number of the erythrocytes in this disease, immature cells in the peripheral blood present evidence of increased activity of the bone marrow, an appearance usually associated with anemia. The occurrence of polychromatophilia, of normoblasts and of myelocytes in the blood cannot be explained satisfactorily in any other way. Studies have shown that the stomach of man and of dogs secretes a substance which may be recovered in a form suitable for intramuscular injection and which is a powerful stimulant of marrow activity in pernicious anemia.1From the stomach contents of swine and cattle, obtained immediately after slaughtering, a similar, probably identical, substance has been obtained. This substance is dialyzable through collodion and withstands esterification, properties that would seem to remove it from the enzymes. It is probably