A low-nicotine cigarette smoke condensate, 12 fractions of the condensate, and a reconstituted sample were tested for their ability to induce transformation in the mouse cell line 3H/10T-1/2 CL-8. This cell line is noted for its remarkable low spontaneous rate of transformation. Both the crude condensate and the reconstituted sample as well as two specific fractions induced transformation in the mouse cells. These transformed cells produced fibrosarcomas when injected s.c. into antithymocyte serum-treated syngeneic mice.