Abstract
Enclosure of the genome of Schmidt-Ruppin strain of RSV [Rous sarcoma virus] within the viral envelope of RAV [Rous associated virus]-l rendered it inactive in tumor induction in newborn hamsters. Among the variants of Bryan strain of RSV coated by RAV-1, RAV-2, and a newly Isolated helper virus CRAV-50), only the last variant produced sarcomas In hamsters. The results are consistent with the hypothesis that the ability of RSV to induce mammalian tumors is determined by the viral envelope.