The erythropoietin content in cord blood was determined from normal full term infants, fants, from infants born more than 2 weeks after term, from infants of preeclamptic, diabetic and Rh-immunized mothers. The group of normal infants showed higher ergthropoietin levels than the premature group. Highly elevated erythropoietin levels were found in some infants in the preeclamptic, the postmature group and in the infants of diabetic mothers. The greatest frequency of increased erythropoietin levels were found in the preeclamptic group. The infants with the greatest rise in erythropoietin content often showed clinical signs of dysmaturity. Erythroblastotic infants seemed to show a rise in erythropoietin levels when capillary hemoglobin fell below 13-11 g. The results indicate that both anemic and hypoxic hypoxia may give an increase in erythro-poietin content, and that erythropoietin is a stimulating factor for red cell production in fetal life, at least in the last months.