The incorporation of technologies into cigarettes such as filters, filter ventilation, porous cigarette papers, expanded tobacco and reconstituted tobacco sheet has resulted in cigarettes with a wide range of "tar" yields. The objectives of this study were to characterize the US cigarette market according to "tar" category (i.e. full flavor, FF; full flavor low tar, FFLT; or ultra low tar, ULT) and to determine whether the Kentucky reference cigarettes K1R4F and K1R5F are representative of FFLT and ULT cigarettes, respectively. As a means of characterization and comparison, the mainstream smoke from a representative sample of commercially available cigarettes from each market segment and the K1R4F and K1R5F Kentucky reference cigarettes was analyzed for the presence and level of 18 selected chemical constituents. In addition, a measure of the mutagenic activity of the mainstream smoke condensate from these cigarettes was determined using an Ames Salmonella mutagenicity assay. All cigarettes were smoked according to US Federal Trade Commission (FTC) guidelines. Results indicated that, overall, mainstream smoke constituent levels are well predicted by FTC "tar" yield--constituent levels increased as "tar" delivery increased. Based on the selected analytes measured in mainstream smoke, the K1R4F reference cigarette was generally representative of the FFLT segment of the US cigarette market. The K1R5F reference cigarette was representative of the ULT segment of the US cigarette market for cigarettes with "tar" deliveries approximate to it. In terms of mutagenic activity, a direct relationship was also demonstrated on a per cigarette basis-revertants per cigarette increased with increasing "tar" delivery. There was a weak tendency (R-square = 0.12, P = 0.08) for specific activity (revertants/mg "tar") to increase with decreasing "tar" yield-lower "tar" products had a slightly higher specific activity. No significant differences (P > 0.05) were observed when the specific activities of the condensates from the K1R4F and K1R5F reference cigarettes were compared to the market segments that they were designed to represent, FFLT and ULT, respectively. Overall, these results support the use of the K1R4F and the K1R5F as acceptable reference cigarettes for comparative mutagenicity and smoke chemistry studies of cigarettes available on the US market.