Previous work has shown that food contains a large number of minor dietary constituents that can inhibit the occurrence of cancer. Additional inhibitors from four different natural sources will be the subject of this presentation. 1. Citrus fruit oils. Orange, tangerine, lemon, and grapefruit oils given p.o. induce increased glutathione (GSH) S-transferase activity in tissues of the mouse. When fed in the diet prior to and during the course of administration of benzo(a)pyrene (BP), the four citrus fruit oils inhibit formation of tumors of both the forestomach and lungs of mice. When fed either before or after the administration of 7,12-dimethylbenz(a)anthracene (DMBA) orange oil inhibits mammary tumor formation. 2. Garlic oil. Allyl methyl trisulfide (AMT), a constituent of garlic oil, has been synthesized recently. When given p.o. 96 and 48 hr prior to BP, AMT inhibits the occurrence of forestomach tumors in mice. 3. Green coffee beans. Two diterpene esters, kahweol palmitate and cafestol palmitate, which are potent inducers of GSH S-transferase activity have been isolated from coffee beans. When administered p.o. prior to DMBA the two diterpene esters inhibit mammary tumor formation. 4. Cruciferous vegetables. Several glucosinolates occur in cruciferous vegetables. Efficient procedures for the isolation of these compounds have been developed recently. The inhibitory effects of three of these i.e. glucobrassicin, glucotropaeolin, and glucosinalbin were studied in several animal models. Glucobrassicin caused substantial inhibition of BP-induced neoplasia of the lung and forestomach of the mouse and DMBA-induced neoplasia of the breast in rats. Glucosinalbin and glucotropaeolin are less active in these systems. In addition to protective effects, indoles derived from the hydrolysis of glucobrassicin have potential harmful properties. The implications of multiple properties and factors which may determine their consequence will be discussed.