Myeloperoxidase, the most abundant enzyme in neutrophils, catalyses the conversion of hydrogen peroxide and chloride to hypochlorous acid. This potent oxidant has the potential to cause considerable tissue damage in many inflammatory diseases. We have investigated the ability of dapsone, diclofenac, primaquine, sulfapyridine and benzocaine to inhibit hypochlorous acid production by stimulated human neutrophils. The drugs were also tested against purified myeloperoxidase using xanthine oxidase to generate hydrogen peroxide and superoxide. The inhibitory effects of the drugs on hypochlorous acid production, either by cells stimulated with phorbol myristate acetate or by myeloperoxidase and xanthine oxidase, were significantly less than those determined with myeloperoxidase and reagent hydrogen peroxide. Comparable potency was observed only when superoxide dismutase was present to remove superoxide. We also observed that with the xanthine oxidase system, inhibition of hypochlorous acid production by dapsone decreased markedly as the concentration of myeloperoxidase increased. Dapsone was a poor inhibitor of hypochlorous acid production by neutrophils stimulated with opsonized zymosan, regardless of the presence of superoxide dismutase. With this phagocytic stimulus, catalase inhibited hypochlorous acid formation by only 60%, which indicates that a substantial amount of the hypochlorous acid detected originated from within phagosomes. Thus, it is apparent that dapsone is unable to affect intraphagosomal conversion of hydrogen peroxide to hypochlorous acid. All the drugs inhibit myeloperoxidase reversibly by trapping it as its inactive redox intermediate, compound II. We propose that superoxide limits the potency of the drugs by reducing compound II back to the active enzyme. Furthermore, under conditions where the activity of myeloperoxidase exceeds that of the hydrogen peroxide-generating system, which is most likely to occur in phagosomes, partial inhibition of myeloperoxidase need not affect hypochlorous acid production. We conclude that drugs that inhibit myeloperoxidase by converting it to compound II are unlikely to be effective against hypochlorous acid-mediating tissue damage.