Monoclonal antibodies directed at the proteins of murine leukemia viruses (MuLVs) have some value as immunological reagents, but differ greatly in their applicability. The kit described in this invention uses a monoclonal antibody designated 83A25, which identifies almost all ecotropic, xenotropic, polytropic, and amphotropic MuLVs. It can be used in a wide variety of procedures, including focal immunofluorescence assays on live or fixed monolayers, immunoblotting, immunoprecipitation, immunohistochemical, and flow cytometric procedures.