Anti-mouse MAC-3 monoclonal antibody recognizes the MAC-3 antigen which is found on macrophages and some nonlymphoid tissues, but not on lymphocytes. It is a glycoprotein showing a broad band in SDS-PAGE, and is synthesized by macrophages (average molecular weight 92-110,000 daltons, depending on the origin of macrophages). It reacts with peritoneal exudate macrophages where the exudate is provoked by thioglycollate, protease peptone, L. monocytogenes, lipopolysaccharide and con- canavalin A. It does not react with thymocytes, spleen, lymph node or bone mar- row cells in immunofluorescent flow cytometry. MAC-3 is a general marker for macrophages and can be used to distinguish these cells from lymphocytes. Its Mr 110,000 and cell distribution differentiate MAC-3 from MAC-2 and MAC-1 antigens.