The kinetics of erythroblast proliferation were studied by means of quantitative 14C-autoradiography in 5 patients showing anemia due to infection or malignancy, in 7 patients suffering from iron deficiency anemia, and in two individuals with bleeding enemia. Compared with a group of 5 healthy persons a markedly reduced turnover of erythroblasts was found in the anemia due to infection, malignancy, and iron deficiency, whereas this turnover was normal or increased in the case of bleeding anemia. The reduction is caused by a progressively decreasing rate of erythroblast proliferation and maturation with advancing development into mature cells. No indications of a change in the number of cell divisions were found in the anemia of infection, malignancy, and of iron deficiency, nor was there evidence of an intramedullary death of nucleated red cell percursors. The imbalance between production and loss of red cells causing anemia shows a different pattern in the 3 groups of disease: In bleeding anemia the insufficiency of supply is not yet apparent from the rate of erythroblast turnover giving weight to the factor of blood loss. In anemia due to infection, malignancy, and iron deficiency the but moderately increased rate of red cell destruction cannot be compensated because of several impairments: The rate of erythroblast turnover is reduced, and, in addition, a moderate portion of maturing cells is destroyed, probably at the reticulocyte stage. As the most significant factor, the bone marrow is unable to compensate the anemia by an effective erythroblast hyperplasia. In iron deficiency this hyperplasia is inadequately low, in infection and malignancies, however, it is more or less missing.