Splenic erythroblasts obtained from mice during the acute disease caused by either the polycythemia-inducing (FVP) or anemia-inducing (FVA) strain of Friend virus were examined for their degree of terminal differentiation. Morphology, benzidine staining, and heme synthesis kinetics showed that many erythroblasts from FVP-infected mice were undergoing terminal differentiation, while few erythroblasts from FVA-infected mice showed evidence of terminal differentiation. When cultured in methylcellulose medium, splenic erythroblasts from FVP-infected mice completed differentiation without the addition of erythropoietin (EP) to the medium. However, splenic erythroblasts from FVA-infected mice underwent terminal differentiation in vitro only when EP was added to the medium. From spleens of FVA-infected mice, a population of large, immature-appearing erythroblasts was obtained by separation with velocity sedimentation at unit gravity. Serial studies of the separated erythroblasts which were cultured with EP showed that despite some heterogeneity in their proliferative capacity, they were relatively homogeneous in their degree of differentiation in that they had not begun to synthesize heme or globin. Morphological changes and syntheses of heme and globins were monitored during terminal differentiation induced in vitro by EP. The accumulation of immature erythroblasts in vivo, their responsiveness in vitro to EP, and availability of large numbers of cells (108 or more) make the splenic erythroblasts of FVA-infected mice an ideal population of cells with which to study EP-mediated terminal differentiation. This erythroblast population should permit the biochemical and molecular studies in erythroid differentiation which heretofore had to be done with chemically induced erythroid differentiation in continuous cell lines.