1. Cytochrome P450 isozymes from Asian (31 Chinese subjects) and Caucasian (14 Finnish subjects) livers were examined for their roles in the metabolism of toluene (rates of benzyl alcohol, o- and p-cresol formation) and styrene (rates of styrene glycol formation). 2. For toluene, the overall rate of metabolism was higher in samples from Finnish than from Chinese subjects. At 0-20 mM toluene, the rate of o-cresol formation was significantly higher in Finnish microsomes than in Chinese ones. The formation rates of benzyl alcohol and p-cresol in Finnish samples were also higher than those of Chinese samples, but only at a high substrate concentration (5.0 mM). For styrene metabolism, the Chinese liver microsomes showed higher metabolic rates than the Finnish ones at 0.085 mM styrene, but not at the higher substrate concentration. 3. Mean expression levels of immunochemically detected CYP1A2/1 and CYP2B6 were almost 3-fold higher in Finnish microsomes, whereas CYP2E1 was 1.7-fold higher in Chinese samples. 4. Correlation analysis showed that CYP2E1 (benzyl alcohol formation) and CYP1A2/1 (o-cresol formation) contributed to the metabolism of toluene at the low substrate concentration, whereas CYP2C8 was the form more actively involved at the higher toluene concentrations. At the higher concentration (1.8 mM) of styrene, CYP2B6 was most active isozyme to catalyse the formation of styrene oxide from styrene. 5. These results suggest that CYP2E1 and CYP1A2/1 are the main isoforms responsible for the metabolism of toluene at low substrate concentrations in human liver microsomes, CYP2E1 at low styrene concentration, and CYP2C8 and CYP2B6 at high concentrations of toluene and styrene respectively.