The molecular structure of murine type-C RNA leukemia viruses has been studied using the Friend strain of spleen focus-forming virus (SFFV) which causes a rapid erythroleukemia; and using the AKR-MCF (mink cell focus-inducing) virus, which causes a thymic leukemia. Recent evidence obtained with these two murine leukemia virus systems suggests that recombination between helper-independent ecotropic virus and genetic sequences highly related to mouse xenotropic virus may be important in the induction of leukemia. Analysis of the recombinant viruses by tryptic peptide analysis of viral gp70, have shown that the recombination occurs within the env gene region. These results indicate that after infection of the host cells by the ecotropic virus, a genetic recombinational event may occur between the ecotropic env gene sequences and endogenous env sequences highly related to xenotropic virus leading to the formation of a leukemogenic virus.