Since reactive oxygen radicals play an important role in carcinogenesis and other human disease states, antioxidants present in consumable fruits, vegetables, and beverages have received considerable attention as cancer chemopreventive agents. Thus, in order to identify antioxidants in plant extracts, test materials were assessed for potential to scavenge stable 1,2-diphenyl-2-picrylhydrazyl (DPPH) free radicals, reduce TPA-induced free radical formation in cultured HL-60 human leukemia cells, and inhibit responses observed with a xanthine/xanthine oxidase assay system. Approximately 700 plant extracts were evaluated, and 28 were found to be active in the DPPH free radical scavenging assay. Based on secondary analyses performed to assess inhibition of 7,12-dimethylbenz(a)anthracene-induced preneoplastic lesion formation with a mouse mammary organ culture model, Chorizanthe diffusa Benth. (Polygonaceae), Mezoneuron cucullatum Roxb. (Leguminosae), Cerbera manghas L. (Apocynaceae) and Daphniphyllum calycinum Benth. (Daphniphyllaceae) were selected and subjected to bioassay-guided fractionation. 5,7,3',5'-Tetrahydroxy-8,4'-dimethoxyflavonol, 5,8,4'-trihydroxy-7,3'-dimethoxyflavonol, 5,3',4'-trihydroxy-7-methoxyflavonol, and 6,3',4'-trihydroxy-7-methoxyflavonol were identified as active principles from C. diffusa. Piceatannol, trans-resveratrol, apigenin and scirpusin A were found as the active principles of M. cucullatum, olivil, (-)-carinol, and (+)-cycloolivil were active principles from C. manghas, and 5,6,7,4'-tetrahydroxyflavone 3-O-rutinoside and kaempferol 3-O-neohesperidoside were active principles from D. calycinum. Of these substances, the hydroxystilbenes piceatannol and transresveratrol have thus far been shown to inhibit carcinogen-induced preneoplastic lesion formation in the mouse mammary gland organ culture model.