Murine leukemia viruses were previously demonstrated to be able to infect efficiently non-virus-expressing tumors in vivo. In the present study the infectivity and tissue distribution of Friend murine leukemia virus (F-MuLV) in normal and tumor-bearing C57BL/6J (B6) mice were examined. Two syngeneic fibrosarcoma-inducing cell lines were used: Cells from a 3-methylcholanthrene-induced fibrosarcoma syngeneic to B6 mice (MCA-FS) and cells from a Harvey murine sarcoma virus-transformed, nonproducer sarcoma syngeneic to B6 mice (H-NP) were described in the preceding study. Both cell lines lacked ecotropic viral expression. F-MuLV produced in vitro was rarely able to infect normal adult B6 tissue in vivo and lacked pathogenic potential. Adult animals receiving F-MuLV remained clinically normal during 20 months of follow-up and had no detectable viremia, although some had persistently infected thymuses and long bones. In animals receiving a single dose of F-MuLV given to superinfect either the MCA-FS or the H-NP induced tumors, virion antigens were found only in tumor tissue and not in the normal host organs studied. Infectious virus was abundant in tumors; occasionally, it was found in thymuses and long bones of animals bearing superinfected H-NP tumors but rarely in other organs. Localization of F-MuLV in MCA-FS tumors appeared to be more selective with rare contamination of host organs. The presence of a rescuable sarcoma genome in H-NP may explain the discrepancy between MCA-FS and H-NP tumors. The possibility of increasing the efficiency and selectivity of infection as well as the therapeutic application of this technique are discussed.