The cellular elements of blood obtained from birds with virus-induced erythroblastosis rapidly incorporated radioactivity when incubated in vitro with Fe59Cl3. This effect was not seen until primitive red cells entered the circulating blood. Little of the iron that was incorporated reached heme. Instead most of it was retained by a heat-resistant component which was precipitable with 35% ammonium sulphate and had properties suggestive of a ferritin-like substance. Plasma from leukemic blood had a slight promoting effect on incorporation of Fe59 into normal blood cells in vitro when compared with normal plasma.Blood cells from birds in which the bone marrow was rendered hyperactive with phenylhydrazine also incorporated Fe59 in vitro, a large proportion of this reaching heme. Incorporation into the heme fraction was also observed in blood cells from erythroblastosis-infected birds treated with phenylhydrazine, indicating that heme synthesis was not completely blocked in these birds.
