A series of 16 low-tar cigarettes, yielding from 1 to 10 mg of tar, were smoked on a modified cigarette smoking machine that collected both mainstream (MS, inhaled) smoke and sidestream (SS, between puffs) smoke. The SS smoke is the major contributor to environmental tobacco smoke. The collected MS and SS smoke condensates were evaluated for mutagenicity by the Ames test and compared with MS and SS smoke condensates from a high-tar cigarette. Both MS and SS condensates of low-tar cigarettes (LTCs) were tested with the Salmonella strains TA1538 and TA100. Except for three cigarettes, the MS smoke mutagenicities of the LTC smoke condensates were significantly reduced (about 30%) when compared with a control, high-tar (23-mg) cigarette. Opposite results were obtained for the SS smoke condensates, which were more mutagenic (about 20%) than the SS smoke condensate of the high-tar cigarette. Thus, LTC mainstream smoke may be less hazardous to the LTC smoker, whereas LTC sidestream may emit more mutagenic compounds into environmental tobacco smoke, which, through passive inhalation, could affect both smokers and nonsmokers.