The purpose of this study was to determine whether an inappropriately low erythropoietin response in premature infants might be a basis for the anemia of prematurity. Erythropoietin was measured by radioimmunoassay in conjunction with hemoglobin and reticulocyte count in untransfused premature infants between birth and 60 days of age. The 27 infants had a mean gestational age of 31 weeks and a mean birth weight of 1378 gm. Between 2 and 30 days, mean erythropoietin concentration was 9.7 mU/ml, significantly and substantially lower than 15.2 mU/ml in 15 concurrently studied healthy adults (P less than 0.01). Subsequently, from 30 to 60 days, values rose gradually to a mean of 17.2 mU/ml, which did not differ significantly from the mean value in adults. Hemoglobin values fell from a mean of 12.9 gm/dl during the first month to 9.0 gm/dl between 30 and 60 days. Thus, during the second postnatal month, preterm infants had essentially the same erythropoietin values as in adults despite a mean hemoglobin concentration that averaged less than two thirds the adult value. This failure to mount a greater erythropoietin response may help to explain why hemoglobin declines to such low values at 2 months of age.