We have compared warts and carcinomas from cottontail and domestic rabbits for the presence of cottontail rabbit papillomavirus (CRPV) and the status of the viral DNA genome. Our studies indicate that benign warts from cottontail rabbits, whether found naturally or induced in the laboratory, contain large amounts of virus and on the average 1000 copies of the virus genome per cell. Both benign warts and carcinomas from domestic rabbits contain significantly reduced levels of virus relative to cottontail rabbit warts and an average of 100 copies of the virus genome per cell. A single sample of a naturally occurring cottontail rabbit carcinoma contained approximately 80 copies of the viral genome per cell. None of the tumors that we have analyzed thus far appear to have integrated viral genomes by Southern blot analysis of undigested and restriction endonuclease-digested DNA samples. Furthermore, the CRPV genome present in domestic rabbit carcinomas and a cottontail rabbit carcinoma appears identical by restriction endonuclease mapping to that present in papillomas of cottontail and domestic rabbits indicating that no major deletions or rearrangements of the CRPV genome had occurred during the progression of benign to malignant tumors nor was a variant of wild-type CRPV responsible for this phenomenon. Finally, we have demonstrated morphological transformation in vitro of NIH 3T3 and C127 cells upon infection with purified CRPV and upon transfection with purified CRPV DNA. Furthermore, single cell clones derived from transformed foci contain free forms of CRPV DNA that persist through continued passage in culture. Cells transformed by CRPV grow in soft agar in vitro and produce tumors in athymic nude mice.