Lactoperoxidase, when incubated with increasing amounts of promethazine (P) and promethazine sulfoxide (PO) catalyzes the formation of promethazine sulfoxide accompanied by oxygen consumption. An intermediate radical of PO can be detected by electron spin resonance (ESR). Catalase or superoxide dismutase do not inhibit the reaction while dopamine does. The lactoperoxidase-catalyzed formation of dopaminochrome in the presence of hydrogen peroxide is inhibited by P. Both P and PO inhibit acetyl- and butyrylcholinesterase. Purified enzymes were used throughout the study and horseradish peroxidase but not myeloperoxidase had an activity similar to that of lactoperoxidase.