Eight monoclonal antibodies to cell surface antigens of human monocytes were evaluated as immunologic markers for recognition of macrophages in sections of normal and diseased tissues, using immunoperoxidase and enzyme histochemical techniques. Monoclonal antibodies assessed were PHM2, PHM3, FMC17, FMC32, FMC33, FMC34, OKM1, and 63D3. Sites studied were human bone marrow, blood, lymph node, spleen, thymus, liver, kidney, lung, and peritoneal lavages, rejecting renal allografts containing inflammatory macrophages, and granulomata showing epithelioid and multinucleate giant cell formation. All antibodies bound to at least some tissue macrophages and, except for FMC32 and FMC33 antibodies, which were identically distributed, each antibody had a distinctive tissue distribution. Some antigens were shared by other bone-marrow-derived cells (megakaryocytes and cortical thymocytes), endothelium, epithelium, and dendritic cells. Antigenic differences were also detected between mononuclear phagocytes present at different sites, different stages of differentiation, and likely different states of activation. These studies provide evidence of major antigenic differences between various populations of human mononuclear phagocytes. They therefore indicate the need for careful evaluation of experiments involving the recognition of macrophages in tissue sections and smears based solely on the use of antimonocyte monoclonal antibodies.