SA12 virus, originally isolated from an uninoculated South African vervet monkey kidney culture, was identified as a new member of the simian virus 40 (SV40)-polyoma subgroup of papovaviruses. The virus produced a cytopathic effect with nuclear enlargement in primary rhesus kidney cells. The virion had papovavirus morphology and a diameter of 44 to 45 nm. The DNA of the virus was a circular, double-stranded, superhelical molecule with a mean length 101% that of SV40 DNA and an estimated molecular weight of 3.3 X 10(6). The virus was found to be unrelated to other papovaviruses by neutralization, immune electron microscopy, and immunofluorescence tests with antiviral sera. SA12 virus-infected cells exhibited a capsid antigen, which has recently been found to be common to viruses of the SV40-polyoma subgroup. The virus readily transformed kideny cells from 10-day-old hamsters. Inoculation of transformed cells produced tumors in 3- to 4-week-old hamsters. The T antigens of SA12 and SV40 viruses were strongly and reciprocally cross-reactive. A high proportion of the sera of chacma baboons, Papio ursinus, and a comparatively lower proportion of the sera of vervet monkeys, Cercopithecus pygerythrus, had neutralizing antibodies to SA12 virus