We explored the effect of in vitro phagocytosis and in vivo inflammation on the MPO content of functioning neutrophils and on the ability of these cells to export active MPO into the extracellular environment. After ingestion of staphylococci, neutrophils retained 52% of their MPO and released 8% into the medium in active form; the remaining 40% of their MPO could no longer be detected. During bacterial infection induced by intradermally injecting staphylococci, neutrophils harvested from minced infected lesions contained 52% of the MPO of circulating neutrophils that had not reached the lesions. Extracellular fluid from the lesions contained active MPO secreted by the neutrophils, and concentrations of 10–45 U/ml were detected. These data demonstrate that functioning neutrophils can lose approximately half of their MPO. In vitro, 4%-8% of neutrophilic MPO appears in the extracellular space and 40% is inactivated. In vivo, the MPO content of inflammatory neutrophils also decreases, and MPO appears in the extracellular fluid in active form where it is available to participate in a variety of physiologic processes.