m-Chloroperoxybenzoic acid (CPBA) was tested for its ability to induce the ornithine decarboxylase (ODC) marker of skin tumor promotion. In contrast to benzoyl peroxide, dicumyl peroxide, and 2-butanol peroxide, 5 mg of CPBA applied twice at a 72-h interval induce ODC activity at least as much as 3 micrograms of 12-O-tetradecanoylphorbol-13-acetate (TPA). ODC induction peaks 36 h after a single CPBA treatment but is maximal 5 h after two applications of CPBA at a 48-h interval. The ODC-inducing activity of CPBA is dose dependent and sustained after chronic treatment. In contrast to TPA, two CPBA treatments at 12-24 h intervals produce no refractory state against ODC induction. The mechanism of ODC induction by CPBA is iron dependent. Various hydrolyzable tannins, condensed tannins (CTs) and their monomeric units remarkably inhibit the ODC response to multiple CPBA treatments. At 12 mg, gallic acid, Aleppo gall tannic acid (TA), catechin, and loblolly pine bark CT inhibit the most CPBA-induced ODC activity. Aleppo gall TA is even effective when applied several hours before CPBA. The tumor-promoting activity of CPBA and its inhibition by plant tannins remain to be evaluated.