The chromosome-damaging effects of cigarette sidestream (SS) and mainstream (MS) smoke condensates and a mixture of these were compared in 8-week-old NMRI mice by intraperitoneal administration. Each filtered commercial brand of cigarette was smoked by a smoking machine under the standard conditions, and the separately collected SS and MS smoke condensates were extracted with acetone/methanol as described elsewhere. The extracts were tested before and after treatment of animals with an enzyme inducer (Aroclor 1254) or inhibitor (Metyrapone). Increased formation of micronuclei within polychromatic erythrocytes (PCEs) of femural bone marrow 30 h after injection of the extracts was regarded as being due to a clastogenic effect. Regardless of the type of smoke extract injected, the increased formation of micronuclei was found to be dose dependent. The SS smoke condensate induced approximately 29% more micronuclei than the MS smoke condensate, the difference being significant (P less than 0.01). The overall clastogenicity of a 1:1 mixture of SS and MS smoke condensates was not substantially different from the activity of either SS or MS smoke condensate alone. Pretreatment of animals with Aroclor clearly enhanced the differences between the number of micronucleated PCEs caused by SS versus MS smoke condensate; SS smoke condensate induced 50% more micronuclei than did MS smoke condensate (P less than 0.001). Pretreatment of mice with Metyrapone did not modify appreciably the induction of micronuclei by either type of smoke. These results are discussed with reference to our previous data involving inhalation experiments and the recent issue of passive smoking.