Topical application of either 5 mumols of 3-isobutyl-1-methylxanthine (IBMX) or 0.25 mumol of adenosine cyclic 3':5'-monophosphate (cyclic AMP) to the initiated skin of the mouse prior to each promotion with 8.5 nmols of 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibited the formation of papillomas and carcinomas. Combined treatments including IBMX and cyclic AMP caused additive reduction of the incidence of skin papillomas and carcinomas promoted by TPA. A good correlation was observed between the reduction of the tumor incidence by IBMX and cyclic AMP and their inhibition of TPA-stimulated polyamine, RNA, protein and DNA synthesis. Since repeated treatments with these agents before or after initiation with a single subcarcinogenic dose of 7,12-dimethylbenz[a]anthracene (DMBA) did not alter significantly the development of skin tumors (Curtis et al.; Perchellet and Boutwell), the present results suggest that, in the two-step initiation-promotion protocol, both IBMX and cyclic AMP treatments may decrease specifically the promoting stimulus of skin carcinogenesis. The same doses of IBMX and cyclic AMP inhibited the accumulation of polyamines and the increase in macromolecular synthesis observed in DMBA-treated skin, but their effect on DMBA-induced skin carcinogenesis was dependent upon the protocol used and the dose of carcinogen applied. IBMX and cyclic AMP treatments failed to inhibit the induction of skin tumors by weekly applications of 0.2 mumol of DMBA. In contrast to the inhibitory effect of cyclic AMP treatment, IBMX enhanced the carcinogenic response to a single topical application of 3.6 mumols of DMBA. The opposite effects of these agents on the carcinogenicity of DMBA correlated well with their different alteration of DMBA-induced unscheduled DNA synthesis in vitro. Cyclic AMP (0.5 mM) enhanced, whereas IBMX (0.5 mM) inhibited, the DMBA-induced incorporation of labeled precursor into DNA of isolated epidermal cells during incubation in the presence of hydroxyurea. Therefore, it is suggested that the different modulation of DMBA carcinogenesis by IBMX and cyclic AMP may result from concomitant effects on both the initiating and promoting components of the carcinogen.