The analysis of sister chromatid exchanges (SCEs) in CHO cells was used to investigate the genotoxicity of smoke emissions collected from a residential wood stove. Both the particle-phase and vapour-phase extracts induced statistically significant increases in SCE with and without an exogenous metabolic system. Extracts of an emission sample were fractionated by HPLC into five fractions of increasing polarity. The most potent fractions in inducing SCE were the nonpolar fraction (containing PAHs) and the second most polar fraction; the latter fraction was active also in the absence of S9-mix. According to known concentration and activity of benzo(a)pyrene, B(a)P alone might have been responsible for only about 1% of the SCE-inducing capacity of the volatile phase. For comparison, smoke condensates from different types of cigarettes were studied. The SCE responses produced by the cigarette smoke condensates and by the unfractionated extracts of wood smoke behaved similarly both for the activity range and for the response pattern with exogenous metabolic system. The weight of a cigarette needed to induce a significant increase of SCEs in CHO cells was about 12-110 of the corresponding amount of wood burned.