Using a standardized rosetting technique with IgG-coated ox erythrocytes, avid IgG (Fc) receptors were demonstrated on red cell precursors. The proportion of receptor-positive cells in normal marrows was highest in early precursors and appeared to be lost with maturation. In megaloblastosis, the absolute percentage of early precursors increase, but there is an even greater increase in the proportion of receptor-positive cells. It is proposed that this reflects the degree of maturation arrest. The specificity of the receptor was confirmed by inhibition studies with aggregated human IgG. In contrast, the expression of the C3b “immune adherence” receptor, assessed by IgM-C3b-coated ox erythrocytes, was seen to increase with erythroid maturation. Early megaloblasts, especially in severe megaloblastosis, showed a marked decrease in C3b receptor activity, again in proportion to the level of maturation arrest. The significance and possible function of these receptors is discussed.