Average potential erythrocyte life spans measured by disappearance rates of transfused, isologous Cr⁵¹-labeled cells in mice of several inbred strains and F₁ hybrids varied from 50 to 55 days and the Cr⁵¹ half time from 15 to 20 days. Sprague-Dawley rats had a potential red cell life span of 60 days and an apparent half time of 19 days. Normal life spans for isologous and homologous erythrocytes were measured both early and late after irradiation in isologous bone marrow chimeras. Homologous chimeras, early after irradiation and bone marrow transplantation, exhibited severe anemia and rapid rate of depletion of host-type erythrocytes. In contrast, after 2 or more months of chimerism, life spans of both host- and donor-type erythrocytes were normal. Mice with rat marrow transplants functional for at least 2 months never cleared rat red cells rapidly. Sudden and very rapid disappearance of mouse erythrocytes was seen from many, but not all, such chimeras. Erythrocytes from normal rats were depleted more rapidly in chimeras than in normal rats, the potential life span being shortened by about 10 days. Mouse-grown (chimeric) rat erythrocytes were depleted in heterologous chimeras at a rate identical to that of chimeric cells in normal rats and of normal rat cells in chimeras.