Eugenol is the medicament used routinely as an anodyne and antiseptic in dentistry and a food flavour and fragrance agent. The drug stimulates the superoxide (O2-.) release of guinea pig neutrophils without a lag time. The production rate increases with the drug concentration and reaches a plateau at 5 mM. However, the induction accompanies the cytotoxicity. The stimulation system of O2-. production is sensitive to mild heating (45 degrees C, 15 min). The system proceeds without artifacts which may be mediated by a radical chain reaction with H2O2 and hydroxy radical, since neither catalase, mannitol nor azide changes the rate. Ca2+ and Ni2+ in the medium enhance the activity, but Mg2+ and Zn2+ have no effect. EDTA inhibits completely, suggesting that intracellular metal ions are involved in this system. Phenolic compounds used as dental medicaments other than eugenol act as potent stimulators of the O2-. production, with the following order of potency: thymol greater than eugenol greater than o-cresol. Resorcinol, guaiacol and hexachlorophene show little activity. This order of potency agrees with the order of hydrophobicity of these chemicals and that of the cytotoxicity to neutrophils. The data suggest that phenolic antiseptic drugs bind to the cell surface hydrophobically, trigger the oxygen burst and make the plasma membrane fragile at a high dose of drugs.