Fv-4 is a mouse gene that dominantly confers resistance to infection by ecotropic murine leukemia virus (MuLV). We previously demonstrated that mixed radiation bone marrow chimeras containing Fv-4r-bearing BALB/c-Fv-4Wr (C4W) bone marrow and Fv-4r-bearing C3H/He (C3H) bone marrow grafted into C3H recipient mice (C4W+C3H-->C3H) were resistant to Friend leukemia virus (FLV)-induced leukemogenesis, even when they contained as high as 70% C3H-derived cells. This indicates that FLV-sensitive C3H-derived cells are rendered refractory to infection and/or transformation with FLV when they coexist in mice with Fv-4r-bearing cells. To investigate the mechanism of Fv-4 resistance to FLV-induced leukemogenesis, we first examined the expression of Fv-4r env antigen in the peripheral blood mononuclear cells (PBMC) of these chimeras. The Fv-4r env antigen was present not only on C4W-derived cells, but also on Fv-4r-bearing C3H-derived cells in C4W+C3H-->C3H mixed bone marrow chimeras. The Fv-4r env antigen that binds to the cells surface of C3H cells was found in sera from normal C4W mice, C4W-->C3H chimeras, and C4W+C3H-->C3H mixed chimeras. The serum Fv-4r env antigen binds to ecotropic MuLV receptors, shown by specific binding to transfectant mink cells expressing ecotropic MuLV receptor, but not to parental mink cells. To determine whether the binding of Fv-4r env antigen to the putative MuLV receptors would block FLV infection, C3H thymocytes or spleen cells that had been preincubated with C4W serum were mixed with FLV and the subsequent production of MuLV specific antigens was examined. C3H thymocytes or spleen cells treated with C4W serum became refractory to binding by FLV. These results provide evidence that the Fv-4r env antigen is released from C4W-derived cells in vivo and binds to cells expressing surface receptors for ecotropic MuLV, thereby protecting them from infection with FLV. The implication of these findings for gene therapy of retrovirus-induced disease such as acquired immune deficiency syndrome (AIDS) is discussed.