Epoxide hydrolase activity towards styrene oxide was measured in the microsomal fraction of 20 human fetal livers. The enzymatic activity was 5.60 +/- 0.52 nmol/min/mg (mean +/- SE) which is about 40% of the previously reported value in human adult liver microsomes. No relation between enzymatic activity and fetal age was observed. The kinetics of the enzyme were studied in 6 different livers and found to obey Michaelis-Menten kinetics. The Km ranged between 0.25 and 0.54 mmol/l and Vmax between 7.2 and 16.7 nmol/min/mg microsomal protein. The enzyme was inhibited both by 1,1,1-trichloropropene-2,3-oxide (TCPO; 0.25 mmol/l) and benzo(a)pyrene-4,5-oxide (BPO; 0.2 mmol/l). Those substances inhibited the epoxide hydrolase by 61 and 14%, respectively, at 1 mmol/l styrene oxide. Thus TCPO was considerably more potent as an inhibitor of the fetal liver styrene oxide hydrolase. Lineweaver-Burk plots of the inhibition data revealed that TCPO exerts an uncompetitive mixed type of inhibition.