Phenols isolated from the traditional medicinal plant Picrorhiza kurroa inhibit the release of superoxide anion (O₂⁻) by activated human neutrophils, but leave the phagocytotic capacity intact. Resting neutrophils and resting or activated human lymphocytes are insensitive to these agents. The underlying mechanism of this highly selective activity is investigated. A critical event is the reaction of the phenols with secretory products from the activated neutrophils. The reaction products interfere with the assembly of a functional NADPH-oxidase in the membrane. Analysis of the mode of activation of the phenols reveals two possible pathways. Catechols react directly with reactive oxygen species (ROS) from the oxidative burst. For the activation of the orthomethoxy-substituted catechols the combined activity of ROS and myeloperoxidase (MPO) is obligatory. Catechols with a dimethoxy substitution cannot be activated metabolically by neutrophil-derived ROS.