Epoxide hydrolase and three cytochrome P-450 isozymes were immunochemically determined in microsomes from adult and fetal human liver and tentatively correlated with some enzyme activities. The P-450 isozymes 5, 8 and 9 present in adult liver could not be positively correlated with the total cytochrome P-450 concentration spectrophotometrically determined. In fetal liver microsomes, isozyme 8 could not be detected by either electrophoretic or immunochemical procedures. Isozyme 5 was the major isozyme present in the fetal liver and its concentration increased in close relation with the total P-450 level. As shown previouly, arylhydrocarbon hydroxylase activity was related to the concentration of isozyme 8 in adult liver. In fetal preparations, the absence of isozyme 8 was associated with a very low arylhydrocarbon hydroxylase activity. Aldrin epoxidase and benzphetamine-N-demethylase activities were correlated with isozyme 5 concentration, but with different slopes for adult and fetal microsomes: adult preparations catalyzed these two reactions more efficiently. Conversely, the dehydroepiandrosterone 16β-hydroxylase, also associated with isozyme 5 concentration, was more active in fetal than in adult microsomes. Moreover, if acetanilide hydroxylase increased with isozyme 5 concentration in adult samples, no correlation occurred between activity and P-450 isozyme level in fetal microsomes. Hydroxylations of lauric acid in positions 11 and 12 and of dehydroepiandrosterone in position 16α increased with total P-450 concentration but not with isozyme concentrations whatever the age considered. Lastly, epoxide hydrolase activity toward benzopyrene 4,5-oxide was closely associated with its immunochemically determined level. These results clearly suggest that multiple mechanisms are involved in the regulation of different drug-metabolizing enzymes in the human fetus.