A papovavirus was isolated from uninoculated kidney cell cultures of the Cynomolgus Macaque (Macaca fascicularis), in which it produced extensive cytopathologic changes after prolonged periods of culturing. On basis of virion morphology, genome size, and restriction endonuclease data, this virus was identified as a new isolate of the Stump-Tailed Macaque Virus (STMV) and subsequently named CK-strain (Cynomolgus kidney) of STMV. Without inducing c.p.e. the virus replicated also in BSC-1, VERO, human embryonic, and calf kidney cells. The relationship between STMV and SV40 was investigated by cross-blot hybridization between DNA fragments of both viruses. The region containing the SV40 regulating sequences showed strong homology with STMV DNA. No antigenic relationship between STMV and SV40 could be demonstrated. Antibodies to STMV were not found in sera from 20 Cynomolgus Macaques, but were detected in 25 out of 57 cattle sera and 6 out of 26 bovine colostrum samples. The serologic findings indicate that STMV is not a latent simian virus, but a virus of bovine origin.