The morphological and functional characteristics of human marrow erythrocytes cultured with a recently developed methylcellulose colony assay technique was examined. Erythrocytic cells in various stages of development were observed, and a significant degree of maturational synchrony within individual colonies was noted. By light microscopy, colonies consisting of late normoblasts appeared compact, had an orange hue attributable to their Hb and demonstrated pseudoperoxidase activity, but colonies composed of early erythroblasts grew less compact or in clusters of smaller cell aggregates and showed no reddish tinge. Colonies possessing intermediate features were also observed. Maturational synchrony of individual colonies was confirmed using transmission and scanning EM. The ultrastructure and cytochemistry of most immature cells were normal. The mature erythrocytes were severely microcytic and hypochromic and contained one to several Heinz bodies. These defects in the cytoplasmic maturation of erythrocytes corresponded with impaired granulocytic maturation in culture, observed previously, and suggest environmental or nutritional defects in culture. Linearity of the method was confirmed using 5 normal bone marrows. Erythropoietin dose-responses observed in 10 normal marrows were comparable to the previously reported results and revealed significant variation in individual plating efficiencies.