Macrophages recovered from the peritoneum of mice, 48 h after concanavalin A administration, are primed and have a higher content of myeloperoxidase (MPO) than resident cells. The increase in MPO content is accompanied by an increased capability of macrophages generate hypochlorous acid and increased peroxidase activity. Contrary to the common sense, neutrophils is not the source of the MPO activity found in primed macrophages since macrophages recovered from mice treated with antigranulocyte antibody preserve the peroxidase activity. Given the broad spectrum of action of MPO, the preservation of MPO in primed macrophages might play a special role in the killing of pathogens and inflammation. (C) 2002 Elsevier Science (USA).