The multienzymatic complex known as cytochrome P-450 represents one of the most important hemoprotein families of the liver. It participates in the metabolism of steroids, fatty acids, prostaglandins and liposoluble vitamins, and also plays a role in the bioactivation of xenobiotic compounds (generates reactive metabolites which produce acute and chronic lesions in liver tissue). This is a report on the low concentrations of total microsomal P-450 (0.093 +/- SD = 0.069 nMoles/mg protein) found in liver biopsies of 19 mexican patients diagnosed as having biliary lithiasis, as compared to an anglosaxon population with the same liver pathology (0.415 +/- 0.105 nMoles/mg protein). These low values are in agreement with the observation of a high incidence in normal mexicans (91.7%) of poor nifedipine metabolizers. Our findings justify an analyses of hepatic RNAm and cDNA of mexican individuals according to ethnic background, diet and environmental contaminants.