A study has been made of the immunospecific, apple-green staining patterns imparted by fluorescent antibody to Rous virus in frozen sections of wing tumors produced in young chickens by various initiating doses of virus. Variations in the cellular site (cytoplasm, cell membrane, and nucleus), the distribution (particulate or homogeneous), and the quantity (little or much) of Rous viral antigens were seen. The per cent of sarcoma cells containing viral antigens was related to the tumor initiating dose of virus and to the infectivity titer of virus extracted from the wing tumor. With certain assumptions it was estimated that a quantity of virus approaching one tumor-producing dose (1 TD(50)) per tissue section was detectable with fluorescent antibody. Sarcoma cells containing viral antigens were identified as early as 4 days after inoculation of Rous virus. At this time viral antigens were also localized in the skeletal muscle of the inoculated wing in a characteristic focal pattern of deposition near or on the sarcolemma. This association between viral antigens and skeletal muscle fibers was observed also at 3 days after virus inoculation, when sarcoma cells were not yet demonstrable in the tissue sections. Viral antigens were detected in sarcoma cells in lung tumors of young chickens and in tumors of the chorioallantoic membrane of chick embryos.