Skin tumor promotion after a short-term exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA) was studied in female SENCAR mice. Mice were dosed once by the topical application of 20 micrograms of dimethylbenz[a]anthracene (DMBA) in 0.2 ml acetone. A week later, they received topical applications of TPA (2 or 4 micrograms per 0.2 ml acetone) once or twice a week for periods of 1-10 weeks and were killed at 30 weeks. Skin tumors were counted and measured for size weekly. When TPA was applied once a week for 10 weeks or only twice a week for 2 weeks, there was significant promotion of papilloma formation in a large proportion of mice initiated with DMBA. Mice that received one or two applications had a few skin tumors. The total number of papillomas decreased considerably and the majority appeared to regress after 20 weeks in mice that received TPA treatment for 10 weeks. In mice that received only 4 TPA treatments, however, the majority of the papillomas grew progressively in size and did not regress during the entire experimental period. A greater proportion of these tumors progressed to carcinoma than did those in mice receiving TPA for 10 weeks. Thus, a short-term exposure was effective in causing certain changes in skin of SENCAR mice that led to tumor development and progression.