In order to characterize chronically accelerated erythropoiesis, we studied the ultrastructure of bone marrow and spleen of ha/ha and sph/sph mice, two mutants with profound hemolytic anemia secondary to deficiency of the erythrocyte membrane protein spectrin. The marrows and spleens of both varieties were extremely erythropoietic and were without histological abnormalities directly related to spectrin deficiency. Erythropoiesis was consistently associated with distinctive, dark branched cells which constituted large proportions of the stroma of the mutant spleens and marrow. These dark cells were not present in untreated and acutely bled controls. Plasma clot assays for erythroid progenitors revealed that CFU-E concentrations in the mutant marrows were significantly increased over those in untreated controls while BFU-E concentrations were approximately half. In addition, mutant CFU-E often gave rise to abnormal appearing colonies. Spectrin, though crucial to erythrocyte function is probably not important to the process of erythroid differentiation and maturation. The status of erythroid precursors in the marrows of the spectrin deficient mice is similar to that of mice subjected to an acute bleed. The divergent changes in CFU-E and BFU-E may indicate that these two cells play different roles in accelerated erythropoiesis. The dark cells that we describe are similar to stromal cells observed in models of the early stages of erythropoiesis.