Infection of (C57BL/6 X DBA/2)F1 (BDF1) mice with 2,500 FFU of Friend virus complex (FV) resulted in erythroleukemia followed by recovery, at which time virus could not be detected in the spleens of mice, and with splenomegaly (progressor mice) or without splenomegaly (regressor mice). Progressor and regressor mice developed equally high amounts of FV-neutralizing activity in their sera. Progressor mice contained cells capable of producing virus, despite the lack of viral envelope antigen(s) in their spleens. The tropism of FV recoverable from the spleens of secondary recipients, injected with spleen cells from progressor mice, did not show any change, although the viral genome was present in Fv-I-restrictive host for at least 7 weeks. When the number of spleen colony-generating cells was enumerated, by the spleen colony assay, the frequency in normal syngeneic and in allogeneic hosts was approximately the same at the early stage of erythroleukemia but was about 1,000 times higher in the syngeneic recipients during leukemia progression. These spleen colony-generating cells were considered to be FV-transformed cells capable of self-renewal and capable of generating infectious centers (IC), respectively. Most of these transformed cells might be non-producer leukemia cells.