The fetal blood erythroblast and reticulocyte counts were determined in umbilical cord samples obtained at 17 to 36 weeks' gestation from 127 pregnancies complicated by red blood cell isoimmunization. The reticulocyte count increased linearly with fetal anemia, and the erythroblast count increased exponentially. Significant erythroblastosis was observed only when the hemoglobin concentration deficit was greater than 7 gm/dl. Of the 52 fetuses with a hemoglobin concentration deficit greater than 7 gm/dl, 35 had ultrasonographic evidence of hydrops. These data suggest that medullary hematopoiesis is stimulated by mild anemia and that recruitment of extramedullary sites occurs when anemia is severe. Extensive hepatic erythropoiesis may be the cause of fetal hydrops in red blood cell isoimmunization.