Early, intermediate and late erythroblasts and reticulocytes were studied by electron microscopic morphometry. The volumes of mitochondria, Golgi zone and autophagosomes, as well as the surface areas of membranes of rough endoplasmic reticulum (RER) and of mitochondrial cristae and the numbers of ribosomes per unit volume cytoplasm were calculated. the results revealed 3 phases in erythroid maturation: (1) early and intermediate erythroblasts, (2) late erythroblasts, and (4) reticulocytes. The only significant difference between early and intermediate proliferating erythroblasts was a decrease in the surface area of the RER in the latter. After the last mitotic division in late erythroblasts significant reductions occurred in the RER, the Golgi apparatus, in the mitochondria and the number of ribosomes. The numbers of mitochondria and ribosomes were further reduced at the reticulocyte stage (clustered ribosomes more rapidly than single ones). Morphometric analysis showed no evidence of degradation of erythroid mitochondria whilst they are free in the cytoplasm, but there was some evidence of degradation after their uptake into autophagosomes in late erythroblasts and in reticulocytes.