Addition of thiourea (TU) or dimethylthiourea (DMTU) decreased killing of Staphylococcus aureus, 502A, and decreased concentrations of hydrogen peroxide (H2O2), and hydroxyl radical (.OH), but not superoxide anion (O2-.) or lysozyme concentrations, in mixtures containing human neutrophils in vitro. Addition of TU or DMTU also decreased concentrations of H2O2, .OH, or hypochlorous acid (HOCl) in neutrophil-free mixtures exposed to beta-D-glucose and glucose oxidase, gamma irradiation, or HOCl, respectively. Our results suggest that TU or DMTU can decrease neutrophil-mediated killing of bacteria by inhibiting O2 metabolite-dependent bactericidal mechanisms.