An RNA bound to the reverse transcriptase of Agrobacterium tumefaciens has been isolated and shown to be oncogenic for stem tissues of Datura stramonium grown under axenic conditions. The tumorous nature of the cellular change induced by the infectious rna was demonstrated by serial grafts of tumors on Datura stems and by cultivation of tumorous tissue in vitro on a medium without supplemental auxins and cytokinins. Active cellular proliferation within tissues of Datura stems was a prerequisite for expression of the oncogenic potential of the RNA. Further, infectious RNA was isolated from avirulent and attenuated strains of Agrobacterium tumefaciens including attenuated derivatives of strain AC58 which have been "heat-cured" of the plasmid associated with virulence. It is proposed that the infectious RNA is an essential but not the sole component of the tumor-inducing mechanism of the crown-gall bacterium.