There has been growing interest in recent years in the potential of brassica vegetables (cabbage, cauliflower, brussels sprouts, etc.) as vectors for the introduction of anticarcinogenic compounds in the diet. Indole-3-carbinol, a major indole metabolite present in the cruciferous vegetables, has been found to inhibit various rodent tumours when administered prior to or during carcinogen exposure. In this study, the antitumour promoting potential of indole-3-carbinol was studied in a two-stage mouse skin model of carcinogenesis. The animals were initiated with a single subcarcinogenic dose of DMBA. After one week, 250 microg of indole-3-carbinol was applied topically to each animal prior to promotion with 5 microg TPA twice per week. Tumour development was significantly inhibited in indole-3-carbinol-supplemented animals in terms of cumulative numbers of tumours and average tumours per mouse. About 44% of male and 29% of female mice remained tumour-free in this group at the end of the experiment. A significant delay in the tumour induction time was also observed in indole-3-carbinol-supplemented animals. This evidence suggests that indole-3-carbinol, in the manner and dose given, inhibits the development of tumours in the two-stage mouse skin model of carcinogenesis.