The Shope papilloma virus (SPV) DNA present in SPV-induced benign and malignant rabbit tumors, particularly in the transplantable carcinoma Vx2 and Vx7, was examined with regard to physical states and extent of methylation. Vx2 and Vx7 carcinomas contained 10-22 viral genomes per diploid cell, and domestic and cottontail rabbit papillomas 40-400 and 1000-8000, respectively. The digestion of Vx2 and Vx7 DNA with the restriction enzyme KpnI, which does not cleave SPV DNA, yielded a single virus-specific DNA band about nine times larger than the genome length, but EcoRI, which cuts the circular SPV DNA once, cleaved this DNA to the genome-size fragments. However, three or four weak bands which may contain viral segments linked to cellular sequences were also identified, and at least two were shared by both Vx2 and Vx7 carcinomas. The analysis with a set of MspI and HpaII, which discriminates the methylated DNA sequence -CC*GG-, showed that 10-40% of the sites of viral DNA are methylated in papillomas, 30-80% in primary carcinomas, and more than 90% in the transplantable carcinomas.