Summary.Experiments have been performed on human sternal marrow kept alive in vitro in Locke's solution with and without the addi‐tion of different antipernicious anemia preparationsor of serum from either the same or from another person. These materials have been shown to maintain erythropoietic activity a t various levels.The slight formation of red cells which takes place in Locke's solution comes to a standstill after 8–10 hours, but is resumed later when red cells are hemolyzed and presumably liberate nutri‐tive materials. Folk acid (0.1 yo) will increase the formation of red cells considerably and liver extract even more so. “Foreign” serum has about the same effect, but the subject's own serum causes most rapid erythropoiesis, thus pointing to the existence of a factor peculiar to each individual. A number of pathological cases were, investigated. The “spontaneous” production of the red cells was diminished in untreated pernicious anemia and leukemia, but was normal in various other conditions studied. I n most of the cases, the pathological inarrow responded quite as a normal marrow does when liver of folk acid is added to the nutrient fluid. Only in lymphogranulomatosis benigna was a significant decrease decrease below normal found in the product:on of red cells when these materials were used. Tyrosine increased the effect of liver extract in untreated pernicious anemia and in leukemia but had no added effect, on normal bone‐marrow.The production of red cells from the pathological marrow nour‐ ished by normal serum is slightly decreased below normal in some of the pathological cases, especially in leukemia. I n this disease and in pernicious anemia there was a decreased total function of bone‐marrow plus “native” serum. All pathological marrows, however, were considerably activated by the addition of normal serum to the nutrient fluid. The use of pathological serum with normal marrow results in a significant decrease in red cell produc‐tion over that found when normal marrow is activated by “foreign” normal serum.In patients in whom a decrease was observed in the activity of the bone marrow cells and of the serum, i. e. in pernicious anemia, effective therapy resulted in an increase in the amount of those substances n both cells and serum which activate the formation of red blood cells in the marrow cultures.These results considered together with the results found in investigations of the ripening of the reticulocytes in vitro show that there are certain active principles in the serum, some being necessary for the production of red cells and some for the ripening of the reticulocytes. It appears that certain of these principles are necessary for both processes.I extend my deep gratitude to the Carlsberg Foundation and to the King Christian the Tenth Pund for supporting these in‐vestigations.