Iodination stimulators, such as the dehydrogenation polymer of caffeic acid (DHP-CA), a protein-bound polysaccharide (PSK), and a commercially available tannic acid, potently inhibited the luciferin-dependent chemiluminescence (LDCL) generated by opsonized zymosan-stimulated human peripheral blood polymorphonuclear cells (PMN). Continuous presence of these substances was necessary to express their inhibitory activity. The extent of inhibition paralleled their ability to scavenge the chemiluminescence generated by the xanthine-xanthine oxidase reaction. They also scavenged the chemiluminescence generated by potassium superoxide solution, but less effectively. An electron paramagnetic resonance spin-trapping technique revealed that DHP-CA significantly, but incompletely, scavenged O2-. The results suggest that O2- might be scavenged both directly by iodination stimulators, and by other oxygen radicals produced by activation of myeloperoxidase-mediated reaction.