Highly lymphomagenic mouse type-C viruses were generated from radiation- or chemically-induced T-cell lymphoma cell lines of NFS/N mouse origin infected with a non-oncogenic ecotropic virus E4. By analysis of these progeny viruses, the following results were obtained. 1) The viruses were lymphomagenic in neonatally inoculated NFS/N and C3H/He mice and W/Fu rats but not in Balb/c and C57BL/6N mice, indicating that they possess the Fv-1n tropism of exogenously infected parent virus. 2) Lymphomagenic viruses consisted of plural viral subpopulations. Recombinant mink cell focus-inducing (MCF) and ecotropic viruses were cloned from them. Inoculation of either MCF or ecotropic virus alone or both viruses together did not cause lymphoma in NFS/N mice and there was no evidence of viral replication in the recipients. 3) Inoculation of either MCF- or ecotropic virus-infected NFS-ME cells alone did not cause lymphoma development in pre-irradiated NFS/N mice, while transplantation of both MCF- and ecotropic virus-infected NFS-ME cells resulted in the development of lymphomas of host origin. These results show that lymphomagenic MCF virus was generated through the recombination of E4 viral genome and a modified proviral DNA of endogenous viruses present in radiation- or chemically-induced lymphomas, and that an interaction or synergism of MCF and ecotropic viruses is required for MCF virus to exert lymphomagenic activity.