A morphologically flat revertant of mink cells nonproductively infected with Moloney sarcoma virus exhibited contact inhibition and lacked detectable sarcoma virus RNA. Superinfection by usually nontransforming type C mammalian leukemia-causing viruses induced transformation and increased sarcoma virus RNA. The results suggest a model for leukemogenesis in animals by increasing, during replication of usually nontransforming leukemia viruses, the levels of RNA from potentially oncogenic cell or integrated virus transforming genes.