The oxidative burst of polymorphonuclear neutrophils (PMN) and their ability to inhibit Paracoccidioides brasiliensis growth was studied in susceptible (B10.A) and resistant (A/J) mice. The cells were obtained after subcutaneous inoculation in air pouches, yielding highly pure PMN preparations; the number of cells was similar for both strains at 24 h and five times higher in the resistant strain at 15 days. The oxidative metabolism of these PMN was evaluated by the luminol and lucigen-enhanced chemiluminescence upon stimulation with PMA or killed P. brasiliensis (Pb). At 24 h of infection PMN from both strains showed similar responses. However, at 15 days a great enhancement of the Pb-stimulated luminol-enhanced chemiluminescence was observed only in PMN from resistant mice. Such increase was markedly inhibited by the addition of catalase. Independent of the mouse strain or time of infection of lucigen-enhanced chemiluminescence showed the same intensity. The lucigen-enhanced chemiluminescence of PMN without stimuli from resistant mice did not change with the time of infection, however, after 15 days of infection a significantly lower chemiluminescence was detected with PMN from susceptible mice. At 15 days of infection the PMN from B10.A were unable to kill P. brasiliensis yeast cells in vitro. Because the lucigenin- and luminol-enhanced chemiluminescence detects, respectively, the O2- production and the myeloperoxidase/hydrogen peroxide halide system, the present data show parallels between deficiency in the production of oxygen-reactive species by PMN and lower fungicidal activity.