We report evidence of a novel mechanism by which polychlorinated biphenyls might act as potent inducers of inflammation. Aroclor 1242 (A1242), a polychlorinated biphenyl mixture, and 2,2',4,4'-tetrachlorobiphenyl (PCB47), a constituent of A1242 that produces the same patterns of effects, impaired the oxidative burst of human neutrophils by inhibiting the antioxidant enzyme superoxide dismutase, which converts O2- to H2O2. Pre-incubation of neutrophils with A1242 or PCB47 before stimulation with phorbol 12-myristate 13-acetate heightened the respiratory burst, producing a significant increase in intracellular O2- production along with a significant decrease in H2O2 production compared with unexposed agonist-stimulated neutrophils. This was also evident in a physiologically relevant situation in which neutrophils pre-incubated with A1242 were subsequently stimulated with a combination of N-formyl-L-methionyl-L-leucyl-L-phenylalanine and tumor necrosis factor-alpha. Incubation of bovine copper-zinc superoxide dismutase (EC 1.15.1.1) with A1242 or PCB47 in a cell-free system reversed the enzyme-mediated inhibition of 6-hydroxydopamine autoxidation, indicating that polychlorinated biphenyls inhibited superoxide dismutase activity. Low superoxide dismutase activity in neutrophils leads to imbalances between production of free radicals and antioxidant defense mechanisms, which can in turn induce tissue damage and hasten the onset of neutrophil apoptosis.