Oral administration of green or black tea inhibited UVB light-induced complete carcinogenesis in the skin of SKH-1 mice. Green tea was a more effective inhibitor than black tea. Oral administration of decaffeinated green or black tea resulted in substantially less inhibitory activity than did administration of the regular teas, and in one experiment, administration of a high-dose level of the decaffeinated teas enhanced the tumorigenic effect of UVB. Oral administration of caffeine alone had a substantial inhibitory effect on UVB-induced carcinogenesis, and adding caffeine to the decaffeinated teas restored the inhibitory effects of these teas on UVB-induced carcinogenesis. In additional studies, topical application of a green tea polyphenol fraction after each UVB application inhibited UVB-induced tumorigenesis. The results indicate that caffeine contributes in an important way to the inhibitory effects of green and black tea on UVB-induced complete carcinogenesis.