We used cells from marrow aspirations that had been performed on 10 infants with the "anemia of prematurity" and tested the responsiveness of their erythroid colony-forming units (CFU-E) to recombinant human erythropoietin. For comparison, we also tested marrow-derived CFU-E from five healthy adults, and circulating CFU-E from cord blood of five healthy neonates. CFU-E from the anemic infants had a 50% maximal response at 0.073 +/- 0.024 U erythropoietin per milliliter (mean +/- SD). They were therefore at least as responsive as were CFU-E from adults, which displayed a 50% maximal response at 0.118 +/- 0.076 U/ml, and as were circulating CFU-E of cord blood origin, which had a 50% maximal response at 0.109 +/- 0.047 U/ml. Because CFU-E from infants with the "anemia of prematurity" appeared highly sensitive to erythropoietin in vitro, we propose that its administration to these patients would likely result in a significant increase in erythrocyte production in vivo.