Erythropoiesis was stimulated in 2- to 5-day-old neonatal rats suckled by phlebotomized mothers. This was established by increases in: hemoglobin levels, mean corpuscular hemoglobin concentration and percentages of peripheral reticulocytes. The oral administration of cow milk containing 4 IU human erythropoietin (Ep) to 10-day-old normal neonates for 4 days induced a reticulocytosis. Significant amounts of Ep appeared in the plasma of 1-day-old rats nursed by mothers bled 3 days prior to delivery as compared to nondetectable levels in 1-day-old control rats of normal mothers. The data suggest that Ep is transmitted to neonatal rats via maternal milk, and escapes inactivation, at least to some extent, in the process of absorption from the gastrointestinal tract with consequent stimulation of erythropoiesis in these animals.