The changes effected by injection of an extract of phospholipids obtained from the plasma of normal human donors (PLE) or an emulsion of commercially available sphingolipids on erythropoiesis in the mouse were studied. The parameters followed were 59Fe uptake by the erythroid tissue and the number of circulating reticulocytes. It was found that in the 12--24-h period following administration of PLE or purified sphingomyelin a significant increase in 59Fe uptake by circulating RBC an by their hemic fraction takes place. This change was associated with a higher 59Fe utilization by the bone marrow and with an increase in the number of circulating reticulocytes.