Organic material from the urine of smokers, coffee drinkers, and controls was extracted and separated into 3 fractions of differing hydrophobicity using preparative reversed-phase high-pressure liquid chromatography. Fractions were assayed for clastogenic activity using Chinese hamster ovary cells. Smoking, coffee drinking, or both habits together resulted in a substantial increase in the genotoxicity of organic material in all 3 fractions. The clastogenicity of fractions 1 and 2 (the two most hydrophilic) was abolished by the addition of either catalase or superoxide dismutase to the Chinese hamster ovary cell system, suggesting the involvement of active oxygen species in the clastogenic response. Clastogenicity of fraction 3, however, was resistant to the action of catalase and superoxide dismutase. Fractions were tested for their ability to generate hydrogen peroxide in vitro during a 10-min incubation at elevated pH. Fractions 2 and 3, but not fraction 1 from smokers, coffee drinkers, or those with both habits generated significantly more hydrogen peroxide at high pH than did the corresponding fractions from control subjects. For fractions 2 and 3 but not 1, the ability of a sample to generate hydrogen peroxide at high pH was positively correlated with its ability to generate chromosome aberrations at neutral pH in tissue culture. The data indicate that both coffee drinking and cigarette smoking result in the appearance of clastogenic materials in urine, and suggest that such clastogenic agents may produce chromosome aberrations via the production of active oxygen species.