The carcinogenicity for mouse skin of condensed volatiles of heated asphalt (8052424) was evaluated. The possibility that either cocarcinogenic or tumor promoting activity existed in any of the chemical fractions which would account for the carcinogenic activity was investigated and an attempt was also made to identify which sets of chemical entities in the several fractions gave rise to a carcinogenic response. Volatiles from heated asphalt were collected and divided into five fractions. Fractions were applied to the skin of male C3H/HeJ-mice or Sencar-mice twice weekly for 104 weeks in proportion to the amount of the fraction in asphalt fumes. Direct carcinogenic activity was limited to two fractions: olefins, which contained alkylated aryl thiophenes and alkylated phenanthrenes; and alkylated phenylethanones and alkylated difuranones. Synergism was not observed by treatment with combined fractions. The authors conclude that, if these fractions can be further subdivided and studied, it may be possible to identify specific compounds and take more active steps toward limiting exposure at the worksite.