The results presented here indicate that erythroleukemia induced by Friend virus evolve by multistage processes. The early phase of Friend disease is characterized by a rapid increase in the number of erythroblastic cells with limited proliferative capacity. The second phase is associated with the appearance of malignant cells with extensive proliferative capacity and autonomous growth. Friend leukemia appears to be a valuable model for studying the sequence of modifications occurring in cells progressing from a normal state to a preneoplastic and finally a highly malignant one.