A method has been described whereby erythropoiesis may be studied in vitro by measuring simultaneously oxygen consumption and the rate of incorporation of radioactive glycine into hemin by rabbit bone marrow. This rate shows only a very slight decrease in the first 20 hours. The optimal pH is about 8.0. The effect of normal serum on this process has been studied. Duplicate determinations show good agreement, but various normal sera show considerable variation in the ability to promote hemin synthesis. Normal serum has been shown to have a marked stimulating effect on hemin synthesis even after considerable dilution. Human serum halts completely the synthesis of hemin by rabbit bone marrow. An interesting dissociations between hemin synthesis and oxygen consumption has been pointed out. This effect is most apparent in experiments on heat-deproteinized serum. This material retains its ability to stimulate oxygen consumption but does not stimulate hemin synthesis at all. The relation of the stimulating effect of normal serum to the hypothetical erythropoietic hormone is discussed.