The in vitro biological oxidation of albendazole to its pharmacologically active sulfoxide and its sulfone by ovine liver microsomes has been studied. Sulfoxidation (maximal rate = 0.412 nmole/min/mg of protein, Michaelis constant = 185 X 10(-6) M) was 107 times more potent than formation of albendazole sulfone. The sulfoxidation corresponds to a reduced nicotinamide-adenine dinucleotide phosphate-dependent enzymatic system characterized by a pH optima value around 8. Flavin adenine dinucleotide-containing monooxygenase could be responsible for this S-oxygenation because of the strong inhibitory effect of methimazole. Albendazole sulfoxidase is inhibited competitively by the related anthelmintic drug fenbendazole (inhibitory constant = 243 X 10(-6) M) and noncompetitively by chlorpromazine (inhibitory constant = 135 X 10(-6) M). At high concentration, chloramphenicol, erythromycin, nalidixic acid, and hexobarbital are less active inhibitors, whereas dexamethasone acetate significantly enhances the reaction which is not inhibited by either carbon monoxide, griseofulvin, imidazole, phenylbutazone, or proadifen.