The 6 methylchrysene isomers, synthesized in greater than 99.9% purity via unambiguous routes, and chrysene were tested for complete carcinogenicity and tumor-initiating activity on mouse skin. 5-Methylchrysene showed a high level of carcinogenicity (comparable to that of benzo[a]pyrene) in contrast to the marginal carcinogenicity of the other methylchrysenes and chrysene. 5-Methylchrysene was also a powerful tumor initiator; 3-methylchrysene had moderate tumor-initiating activity. A method was developed for the isolation of methylchrysenes from cigarette smoke. It involved solvent partitioning of the condensate to obtain the fraction rich in polynuclear aromatic hydrocarbons, followed by column chromatography, a Diels-Alder reaction to remove benz[a]anthracenes, high-speed liquid chromatography to enrich the methylchrysenes, gas chromatography to separate 1-, 2-, 3-, and 4-methylchrysenes, and, finally, paper chromatography to separate 5- and 6-methylchrysenes. For quantitative studies we used14C-5,6-chrysene as internal standard. The smoke of one U.S. blended nonfilter cigarette (85 mm) contained 7.2 ng of 6-methylchrysene, 6.1 ng of 3-methylchrysene, 1.2 ng of 2-methylchrysene, 3.0 ng of 1-methylchrysene, 0.6 ng of 5-methylchrysene, and 36.5 ng of chrysene. Pyrolysis experiments indicated that tobacco sterols were probably not the major source of methylchrysenes in smoke.