Pretreatment of hamster cells with chemical carcinogen enhances transformation by a simian adenovirus, SA7. Such transformants were compared to cells transformed by SA7 alone for the presence of virus-specified antigens, and for the ability to clone in soft agar and to form progressively growing sarcomas in hamsters. The SA7 "T" antigen content and the relative cloning efficiencies in agar of cell lines of both groups were similar. Some cell lines obtained by pretreatment with chemical carcinogen, however, produced a higher incidence of tumors, suggesting that the chemical carcinogen was involved in the transformation of cells by virus. A further indication that these cells differed from cells transformed by virus alone was obtained with MCA-transformed fibroblast cell lines that were subsequently also transformed in vitro by SA7 to cuboidal (SA7 type) morphology. These MCA-SA7-transformed cells produced a higher incidence of sarcomas than cells transformed by virus alone, but a lower tumor incidence than the parental MCA line.