There is evidence that organosulfur compounds can inhibit the induction and growth of cancer. Several organosulfur compounds are dietary constituents and Allium species are a rich source of such molecules. Some but not all epidemiological studies have suggested that consumption of garlic can decrease cancer incidence. There is substantial evidence that constituents of garlic including diallyl sulfides can inhibit the induction of cancer in experimental animals. Effects on both tumor initiation and promotion have been documented. Some effects may be mediated by modulation of carcinogen metabolism involving altered ratios of phase I and phase II drug metabolizing enzymes. 59,60 Inhibitory actions on the growth of tumor cells have been documented and, for some tumor cells, differentiating effects of diallyl sulfides can occur. A definitive mechanism of action has not been established and evidence exists for effects at several sites in carcinogen metabolism and regulation of tumor growth. It is not always clear that laboratory studies can be extrapolated to reasonable levels of consumption by humans of garlic or other Allium species.