Tumors with no detectable virus content, termed “noninfective,” have been found to comprise 10 to 20 percent of those tumors arising below the 50 percent dilution endpoint in 10-day-old chicks inoculated subcutaneously. These tumors did not have a significantly different growth rate when compared to other tumors initiated with equivalent doses of virus. These observations confirm those of Bryan, Calnan, and Mahoney. The cells of noninfective tumors did not produce detectable virus on the chorio-allantoic membrane of the chick embryo or after serial cultivation in tissue culture. Cultures derived from these tumors were resistant to challenge infection with Rous sarcoma virus. Serial transplantation of noninfective tumors resulted in the return of virus-containing tumors. In two instances this did not occur until after four transplant generations. Five noninfective tumors were biopsied and followed; all eventually regressed. Five of 27 virus-producing tumors similarly followed also regressed. There was no correlation between neutralizing antibody levels and virus content, or regression, of tumors. Birds with multiple tumors initiated with less than one tumor-inducing dose of virus were observed carrying both noninfective and virus-producing tumors. These observations were interpreted as suggesting the existence of a form of cell-virus integration in which infectious virus is only rarely produced.