The primary focus of our study is to investigate the biochemical and molecular defense mechanisms against carcinogens and xenobiotics under physiologic conditions. The induction of neoplasm by carcinogens has been well documented in animal models, e.g., benzopyrene (BP) for liver cancer and dimethylbenzanthracene (DMBA) for mammary cancer. We chose to examine the mechanism for carcinogen efflux as the first line of defense against these environmental and endogenous toxins. We proposed that the efflux pump, mediated by a plasma membrane glycoprotein (P-gp) and coded for by mdr1, not only confers resistance to pleotropic cytotoxic drugs but also to chemical carcinogens. Our previous studies demonstrated the P-gp drug efflux pump also mediates carcinogen efflux in human breast and colon cancer cells. Hence, in certain tissues P-gp may constitute an important component of cellular defenses against naturally occurring carcinogens. The identification of naturally occurring effectors of P-gp is relevant to understanding the important physiological role of P-gp in normal cells. We reported that certain flavonoids, kaempferol, quercetin and galangin, enhanced P-gp mediated efflux of Adr, BP and DMBA in human colon and breast cancer cells. To investigate the effects of dietary effectors o drug and carcinogen efflux mechanism in nontumor cells without previous exposure to chemotherapeutic drugs or carcinogen but with P- gp expression, we used Chinese hamster ovary CHO-C9 cells and studied the modulation of drug and carcinogen cytotoxicities. We found that flavonoids which enhanced drug efflux, kaempferol and galangin, markedly decreased the cytotoxicity effect of Adr but not the inactive flavonoids, genistein and apigenin. The IC50 for Adr was increased from 46 nM in controls to 83 and 89 nM with kaempferol and galangin, respectively. In the presence of P-gp inhibitor, verapamil (5mu M), Adr cytotoxicity was enhanced. We further studied the carcinogen cytotoxicity in CHO-C9 cells. Surprisingly, we found the IC50 for DMBA was markedly higher than MCF-7 cells and verapamil decreased the IC50 for DMBA. Our results suggested that certain dietary flavonoids modulated P-gp dependent resistance to drug/carcinogens in nontumor cells. The extreme tolerance for DMBA in CHO-C9 cells may be important in elucidating built-in resistant mechanisms against exposure to environmental carcinogens as a model for cancer prevention.