Although cellular immunity involving activated macrophage is important in resistance to Brucella infections, serum factors and polymorphonuclears (PMNs) play some role in the initial responses to Brucella infections. In this research, we studied respiratory burst of PMNs against opsonized yeast and opsonized inactivated Brucella melitensis in chronic Brucellosis patients and controls with no previous history of Brucellosis. A group of 41 patients and another group of 20 blood donors as control, were included. The other 2 groups included 10 cases and 6 controls. Mean responses of PMNs of patients and controls to opsonized yeast were 110.3 and 129.3 milivolt respectively and the difference was not statistically significant. No statistically significant difference was observed between respiratory burst of PMNs exposed to inactivated Brucella in 10 patients with chronic Brucellosis (Mean 67.2) and 6 control blood donors (Mean 112.5), so we concluded that inactivated Brucella melitensis can't inhibit activity of myeloproxidase enzyme.