The covalent coupling of [3H]LTA4 to human leukocyte LTA4 hydrolase is inhibited in a concentration-dependent fashion by pre-incubation with bestatin. This inhibition correlated with the concentration-dependent inhibition by bestatin of LTB4 and LTB5 synthesis by LTA4 hydrolase. Epibestatin, a diastereomer of bestatin, neither inhibited LTB4 or LTB5 production by LTA4 hydrolase nor prevented the covalent coupling of [3H]LTA4 to the enzyme. In contrast, captopril inhibited both LTB4 synthesis by LTA4 hydrolase and covalent coupling of [3H]LTA4 to the enzyme.