Murine oncovirus antigens represent excellent targets for immune recognition, and virus-associated tumors are generally susceptible to various immunotherapy protocols. Virus-negative tumors, however, are nonimmunogenic and refractory to immunologic control. Therefore, the feasibility of the introduction of antigens onto non-virus-expressing tumors in situ in inbred C57BL/6J mice by systemic administration of nononcogenic murine retroviruses was investigated. Two classes of murine fibrosarcomas were studied: a 3-methylcholanthrene-induced fibrosarcoma syngeneic to C57BL/6 mice (MCA-FS) and a Harvey murine sarcoma virus-transformed, nonproducer fibrosarcoma syngeneic to C57BL/6 mice (H-NP). Both were found to be devoid of infectious ecotropic murine leukemia virus (MuLV) or MuLV antigens. A single dose of Friend murine leukemia virus (F-MuLV) was used to superinfect MCA-FS- and H-NP-induced tumors in vivo and converted these tumors to a highly productive, virus-positive state. In vivo superinfected tumors were indistinguishable from their preinfected counterparts by competition radioimmunoassays for the virion's major envelope glycoprotein, gp71, and its group-specific antigen, p30, and by assays for infectious virus. Analysis of virus from tumor extracts proved that the antigenic specificity of the superinfected tumor was provided by F-MuLV administered systemically to the animals. Finally, an immunoperoxidase technique, applied to tumor cross sections, demonstrated the uniform appearance of viral antigens in the superinfected tumors.