Rice is a staple diet in Asia, where the incidence of breast and colon cancer is markedly below that in the Western world. We investigated potential colon and breast tumor-suppressive properties of rice, testing the hypothesis that rice contains phenols that interfere with the proliferation or colony-forming ability of breast or colon cells. Brown rice, its white milled counterpart, and bran from brown rice were boiled and extracted with ethyl acetate. The extracts were analyzed by high pressure liquid chromatography-mass spectrometry. Eight phenols, protocatechuic acid, p-coumaric acid, caffeic acid, ferulic acid, sinapic acid, vanillic acid, methoxycinnamic acid, and tricin, were identified in the extracts of bran and intact brown rice. These extracts were separated into nine fractions by column chromatography. The effect of bran extract and its fractions at 100 microg/ml on cell viability and colony-forming ability of human-derived breast and colon cell lines was assessed. Bran extract decreased numbers of viable MDA MB 468 and HBL 100 breast cells and colon-derived SW 480 and human colonic epithelial cells as judged by the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4 -sulfophenyl)-2H-tetrazolium assay. It also reduced colony formation of SW 480 colon and MDA MB 468 breast cells. Of the eight phenols identified in the brown rice bran, when applied at 50 microM, caffeic acid decreased numbers of all cell types except HBL 100. Tricin, ferulic acid, and methoxycinnamic acid interfered with cell viability in one or more cell lines. Tricin (50 microM) and the other phenols (200 microM) inhibited colony formation of SW 480 cells. Clonogenicity of MDA MB 468 cells was inhibited by caffeic acid, ferulic acid, and tricin (50 microM). Tricin was the most potent anticlonogenic of the compounds with IC50s of 16 microM in the SW 480 colon cells and 0.6 microM in the MDA MB 468 breast cells. The results suggest that: (a) brown rice and bran contain compounds with putative cancer chemopreventive properties; (b) certain phenols contained in brown rice bran, e.g., tricin, may be associated with this activity; and (c) these phenols are present at much lower levels in white than in brown rice. Thus, the consumption of rice bran or brown rice instead of milled white rice may be advantageous with respect to cancer prevention.