Diallyl sulfide (DAS), a major flavour component of garlic, is known to modulate xenobiotic metabolism and possess antitoxic, bactericidal, antineoplastic, hypolipidemic and hypocholesteromic effects. In the present study, the anticarcinogenic activity of DAS on a 7,12-dimethylbenzanthracene (DMBA)- or benzo[a]pyrene (B(a)P)-induced mouse skin model of carcinogenesis was evaluated. DAS was applied topically either 1 h prior to or 1 h after the administration of DMBA or B(a)P. A significant protection from neoplasia was observed in DAS- and DMBA/B(a)P-exposed animals when DAS was applied topically compared to the animals exposed only to DMBA/B(a)P. In the animals where DAS was applied 1 h prior to the application of DMBA, a lower magnitude of neoplasia was recorded in terms of the cumulative number of tumours and average number of tumours per mouse during the entire period of study (28 weeks) compared to the animals exposed to DAS 1 h later, while in B(a)P-exposed animals, the antitumorigenic potential of DAS was more evident in the mice treated with DAS 1 h after the B(a)P exposure compared to the animals treated with DAS 1 h prior to B(a)P. The antitumour activity of DAS was of a much higher magnitude in B(a)P-induced carcinogenesis in comparison to animals exposed to DMBA in terms of tumour incidence, cumulative number of tumours and average number of tumours per mouse. The results suggest that DAS has a protective effect in PAH-induced mouse skin carcinogenesis.