Induction of gene mutations by SV40 was studied in aneuploid human and Chinese hamster cells. In Chinese hamster cells SV40-induced chromosome aberrations were also studied. SV40 penetrated into the cells of both lines and induced synthesis of the T antigen. The efficiency of infection in Chinese hamster cells was tested additionally by their ability to form colonies in medium lacking the serum growth factor. The maximal number of cells with serum growth factor independence was observed on the first day after infection. When hamster cells had been maintained in "factor-free medium" for the first two passages after infection a sub-line was isolated, which synthesized T antigen 60 days after exposure to SV40. This was considered to be an indirect proof of the integration of viral genome into host chromosome. A significant increase in the frequency of chromosomal aberrations was detected in SV40-infected Chinese hamster cells. It was observed on the first and second days after treatment. The most numberous were the chromosome and chromatid breaks, which were distributed randomly in 5 morphological groups according to the chromosome length. SV40-induced mutations of resistance to 8-AG and 6-MP in human and Chinese hamster cells respectively were detected, when cells were plated in selective medium one to five days after infection. Induction was detected in all the 4 experiments with human cells and in 9 out of 11 experiments with Chinese hamster cells. Induction was highly significant according to the Wilcoxon test (P greater than 0.99), when the results of all experiments carried out in human and Chinese hamster cells were summarized. Resistance was stable after prolonged cultivation of 13 isolated clones under non-selective conditions. It is suggested that viral genome integration, gene mutations and chromosomal aberrations may have common molecular mechanisms. The role of gene mutations in virus-induced carcinogenesis is discussed.