Cigarettes of 4 standard brands and 2 filter-tipped brands were smoked. The heptane-soluble fraction of tar from each brand was tested by painting mice with the equivalent of 8.3 cigarettes per day. Because the yield from the filter-tipped cigarettes was one-third as great as from the standard cigarettes, the filter-tipped treated mice received correspondingly less tar. Skin tumors were produced in 41 of 76 mice painted with tar from standard cigarettes and in 15 of 60 mice painted with the product from filter-tipped cigarettes. No significant difference was found among mice painted with tars from the 4 standard brands or between the 2 groups painted with filtered tar. In every group of mice, some of the skin tumors progressed to cancers within the 1-year period.