The studies reported here show that polychromatophilic erythrocytes as seen in the blood film represent young (Group 1) reticulocytes and that their mean diameter is 27% greater than that of adult erythrocytes. Normally, polychromatophilic erythrocytes constitute 5% or less of the circulating reticulocyte population. With moderate hypoxia or anemia they increase to between 10 and 20%; with severe anemia their proportion increases to more than 20%. However, no such increases are observed in patients whose anemia is unaccompanied by elevated erythropoietin (as in the usual patient with renal failure). In diseases of the marrow stroma, the increase in polychromatophilic erythrocytes appears disproportionate to the degree of anemia present. Since such stromal disease is readily distinguishable, it is concluded that polychromatophilic erythocytes in the peripheral blood film provide a simple means of identifying increased erythropoietin stimulation of the erythroid marrow.