Some epidemiological studies have suggested that exposure to an alternating magnetic field may increase the incidence of some cancers. Our earlier study of carcinogenesis in mouse skin, indicated that exposure to a magnetic field (MF) alone did not promote the growth of tumors. In the present experiment, the ability of a MF to act as a tumor copromoter was investigated. The dorsal skins of female SENCAR mice (6-7-weeks-old) were treated with 10 nmol of 7,12-dimethylbenzanthracene (DMBA) to initiate the carcinogenic process and then tumor development was promoted, for 23 weeks, by weekly applications of 4.9 nmol (0.3 microgram) of 12-0-tetradecanoylphorbal-13-acetate (TPA). One group of 48 mice were exposed to a 60-Hz magnetic field of 2 mT (20 Gauss) for 6 h/day 5 days/week, while a similar group (48 mice) were sham exposed. After week 12, the percentage of mice with tumors and the mean number of tumors per mouse, were higher for the group exposed to MF. At week 18, for example, where the differences between field and sham groups were statistically significant, the percentage of mice with tumors were, respectively, 25% and 8% (P less than 0.05, Fisher exact) and, the mean yield of tumors 1.9 +/- 0.69 and 0.65 +/- 0.46 (mean +/- S.E.M.) (P less than 0.05, Wilcoxon). At week 23 these differences were no longer statistically significant.