In effort to understand how N or B tropism is determined in murine leukemia virus (MuLV) particles, we analyzed the MuLV produced after dual infection of mouse cells by N- and B-tropic MuLV. The progeny MuLV from such a mixed infection are sensitive to Fv-1 restriction in both N- and B-type cells, but are still highly infectious for mouse cells which do not exhibit Fv-1 restriction. This dual sensitivity to Fv-1 restriction is a phenotypic property of MuLV produced by mixedly infected cells, since individual virus clones derived from this MuLV are either N- or B-tropic. In further experiments, we superinfected murine sarcoma virus (MSV)-transformed cells with mixtures of N- and B-tropic MuLVs. The rescued MSV is restricted in its ability to transforms both N- and B-type cells. The results suggest that N- and B-tropic MuLVs specify different determinants, which are incorporated into virions along with the viral genome and which are the recognition sites for Fv-1 restriction. The presence of a given determinant in a virion renders the virus sensitive to restriction in cells of the opposite Fv-1 type.