(+)-Camptothecin (CPT), a topoisomerase I inhibitor specifically toxic toward S phase cells, was tested topically for its ability to inhibit skin tumor initiation by 7,12-dimethylbenz[a]anthracene (DMBA) and complete tumor promotion by 12-0-tetradecanoylphorbol-13-acetate (TPA) in SENCAR mice. Even though CPT does not prevent the covalent binding of a subcarcinogenic dose of DMBA to DNA, it enhances early inhibition of DNA synthesis caused by this initiator and may decrease the essential role of DNA replication in tumor initiation. Indeed, CPT (400 nmol) applied 4 h before or 1 h after DMBA inhibits the yield, but not the incidence, of skin tumors initiated by this compound. Moreover, because it inhibits TPA-stimulated DNA synthesis at 16 h when applied 12 h after the tumor promoter, CPT partially decreases tumor initiation when DMBA is applied 16 h after a TPA pretreatment. CPT (400 nmol) applied 1 h before or 4, 12, 24 or 48 h after each promotion treatment with TPA remarkably inhibits the incidence and yield of skin tumors promoted by this agent. CPT delays and inhibits promotion of skin tumors the most when applied 12-24 h after each TPA treatment, at times when it can block the stimulation of DNA synthesis that follows the period of early inhibition caused by TPA. The ability of post-treatments with 25, 100 and 400 nmol CPT to inhibit skin tumor promotion is dose dependent. In the TPA (stage I)-mezerein (stage 2) protocol CPT (400 nmol) post-treatment inhibits both the first and second stages of tumor promotion, related to its ability to decrease the DNA and ornithine decarboxylase responses required for stages 1 and 2 respectively. The classic model of multistage skin carcinogenesis, therefore, may be valuable to determine if novel CPT analogs are more effective than their parent compound at inhibiting tumor initiation, promotion and progression.