Garlic and onion oil have been shown to inhibit chemically induced skin tumor development in mice. In the present study, the effects of diallyl sulfide and diallyl disulfide, oil-soluble constituents of garlic and onion, on 7,12-dimethylbenz(a)anthracene-induced and 12,O-tetradecanoylphorbol-13-acetate-promoted skin tumor formation were examined in SENCAR mice. Topical application of diallyl sulfide or diallyl disulfide significantly inhibited skin papilloma formation from the ninth week of promotion and significantly increased the rate of survival in the murine model. Our findings support earlier evidence that these naturally occurring compounds may be useful for the chemoprevention of certain types of tumors.