Eugenol has recently been associated with the toxic effects of clove cigarettes on human lungs. We have studied the metabolism and adverse effects of eugenol on human polymorphonuclear leukocytes (PMNs). Myeloperoxidase, isolated and purified from human PMNs, catalyzed the oxidation of eugenol to a reactive intermediate which is likely to be a quinone methide. Eosinophil peroxidase, lactoperoxidase, prostaglandin H synthase, horseradish peroxidase, and rat intestinal peroxidase also supported this hydrogen peroxide dependent reaction. Glutathione inhibited the formation of this metabolite, resulting in the formation of glutathione disulfide and a small amount of eugenol-glutathione conjugates. In cellular incubations, phorbol ester stimulated PMNs catalyzed the covalent binding of [3H]eugenol to cellular protein, which was partially inhibitable by azide. Intracellular glutathione levels decreased by 90% over a period of 30 min in phorbol ester stimulated PMNs exposed to 100 microM eugenol compared with decreases of 30% (phorbol ester alone) or 5% (eugenol alone) in control incubations. In addition, eugenol was more cytotoxic to PMNs in the presence of phorbol ester than in its absence, and eugenol inhibited the phorbol ester stimulated oxidative burst in PMNs as reflected by a decrease in oxygen consumption, superoxide formation, and hydrogen peroxide formation. These results suggest that PMNs are capable of activating eugenol to a reactive intermediate and also suggest a mechanism whereby eugenol can potentially interfere with and adversely affect vital PMN functions.