Spleen cells from Rfv-3r/s mice with Friend virus-induced erythroleukemia were analyzed for expression of virus-induced proteins with monoclonal antiviral antibodies and conventional antisera. Leukemic spleen cells, 30-60 d after virus inoculation, expressed decreased amounts of ecotropic Friend murine leukemia helper virus gag- and env-encoded cell surface and intracellular proteins compared with leukemic cells tested 8-10 d after virus inoculation. In contrast, the spleen focus-forming virus-induced protein, gp55, was present on both leukemia cell populations. This difference appeared to be mediated by the humoral antibody response in Rfv-3r/s mice, which could recognize only ecotropic gag and env proteins, and not gp55. A new gp70 molecule cross-reactive with a recombinant Friend mink cell focus-inducing virus was found in large quantities on late leukemic cells. This protein appeared to be derived from a recombinant virus produced during the course of Friend virus infection. The appearance of this new gp70 suggests that recombinant viruses other than spleen focus-forming virus may play a role in Friend virus-induced erythroleukemia.