Catechol (1,2-dihydroxybenzene) is a major phenolic compound present in the co-carcinogenic fraction of cigarette tar. It has been shown to be a potent co-carcinogen with benzo[a]pyrene (BaP) in mouse skin. In this study we have examined the co-carcinogenic and co-initiating activities of catechol with racemic and enantiomeric 7,8-dihydroxy- 7,8-dlhydrobenzo[a]pyrenes (BaP-7,8-diols) in mouse skin. Similar to enhancement of BaP carcinogenesis, repeated concurrent applications of catechol and (± )-BaP-7,8-diol to mouse skin strongly enhanced (± )-BaP-7,8-diol tumor multiplicity and tumor incidence, and decreased latency. Co-application of catechol with the racemic or either of the enantiomers of BaP-7,8-diol in a two-stage initiation-promotion protocol increased the tumor initiating activity of racemic BaP-7,8-dlol, similar to that of BaP, by ∼50%, but had no statistically significant effect on the tumor initiating activity of the (+)- or (-)-enantiomers in mouse skin. Thus, catechol is as potent a co-carcinogen with (± )-BaP-7,8-diol as it Is with BaP. However, as tested here catechol is a weak co-initiator when applied with (±)-BaP-7,8-diol or BaP.