The weakly acidic fraction of cigarette smoke condensate was fractionated by preparative high-pressure liquid chromatography into major subfractions l-IV. Major subfractions II and III were fractionated further into subfractions A-J. Subtractions A-J were tested for cocarcinogenicity on the skin of noninbred Ha:ICR Swiss albino mice by application with 0.003% benzo[a]pyrene. Subtractions A-C and F-J showed significant cocarcinogenic activity; subfractions A, F, and H were the most active. Catechol was a major component of subfraction A and was also detected in subtractions B-D and F. Major components of the other subfractions included hydroquinone (B), coniferyl alcohol (C and H), hydroxyphenyl alcohols (D), alkyl-2-hydroxy- 2-cyclopenten-1-ones (C, D, and F), hydroxyacetophenones (F), phenolic cyano compounds (F), and fatty acids (F). The results demonstrate the importance of catechol as a cocarcinogen in the weakly acidic fraction of cigarette smoke condensate and indicate the presence of other cocarcinogens.—JNCI 1981; 66:163-169.