A suspension of a variant of Rous sarcoma was injected into four adult and eight newborn rhesus monkeys. Seven newborns developed tumors, three of which were diagnosed as fibrosarcomas, in 2 to 6 weeks; none of the adults have tumors after 11 weeks. Virus was demonstrated in two of the tumors by injecting a tumor-suspension preparation into the chick wing-web where tumors subsequently appeared. To the best of our knowledge this is the first time that sarcomas have developed in primates after a virus has been injected.