Morphological transformation in the hamster embryo cell bioassay has been used to study the possibility that carcinogenicity of benzo[alpha]pyrene (BP) is affected by cigarette smoke extract and whether smoke extract will promote transformations initiated by BP. The transformation frequency increases wth increasing concentrations of BP and smoke extract. In experiments with a combined treatment of BP and smoke extract, the transformation rates were higher than expected for all concentrations from experiments with the compounds tested separately. The greatest potentiating effect was found using 0.01 micrograms/ml BP and 1 microgram/ml smoke extract. The transformation frequency obtained with this combination was 4.3% compared to 1.4% and zero respectively for the individual substances. In experiments where cells were treated sequentially with BP (0.05 micrograms/ml) for 4 days followed by smoke extract (1 or 5 micrograms/ml) for the next 4 days, the transformation frequency was significantly higher than expected on the basis of the compounds tested separately. The demonstration of a synergistic effect between BP and cigarette smoke and the promotion-like effect of smoke extract on BP-initiated transformations of hamster embryo cells are of interest in relation to the higher frequency of lung cancer found in areas with high air pollution compared to rural areas.