The induction of epidermal ornithine decarboxylase (ODC) activity by benzoyl peroxide (BPO) was characterized to evaluate the usefulness of this effect as a short-term marker of BPO-induced mouse skin tumor promotion. The maximal induced levels of ODC specific activity, after a single topical dose of BPO, were greater than 2-fold higher when a cold scraping method was used to prepare epidermis rather than the commonly used heat treatment method. Therefore, the cold scraping method was used for all the work reported here. Application of a single 20 mg dose of BPO to the dorsal skin of SENCAR mice caused a relatively small induction of epidermal ODC activity, to a level less than 1/40 that induced by a single 2 micrograms dose of 12-O-tetradecanoylphorbol-13-acetate (TPA). Furthermore, the time-courses of induction were different after single doses of TPA and BPO, with peak activities observed at approximately 6 and approximately 24 h after treatment respectively. In contrast, after a total of five 20 mg doses of BPO were topically applied (one dose every 2 days), ODC activity was transiently induced to an average level greater than 15 times after a single dose. Additionally, on this dosing regimen, the peak of ODC activity shifted to approximately 4 h after the last treatment, so that the time-course of ODC induction resembled that after multiple applications of TPA. The extent of epidermal ODC induction by BPO was found to be a complex function of the frequency of dosing and the number of treatments. However, when BPO treatments were administered from 1 to 7 days apart, similar maximal induced levels of ODC activity were eventually achieved after application of multiple doses. Importantly, the dose-response for the induction of ODC activity by five doses of BPO (applied one dose every 2 days) was highly correlated with published data on the dose-response for tumor promotion by this organic peroxide, indicating that ODC induction is a good short-term marker of BPO-induced tumor promotion in SENCAR mice.