A monoclonal antibody, termed Ki-M6, is described, which shows a restricted reactivity to cells of the monocyte/macrophage system. On light- and electron-microscopic immunoperoxidase staining Ki-M6 recognizes monocytes and the phagocytosing compartment of macrophages residing in different tissue sites; granulocytes and the so-called immune accessories of B- and T-cell immune response as closely monocyte/macrophage related cell populations do not reveal any reactivity. This is shown by comparison with the monoclonal antibodies Ki-M4 and Ki-M1 or OKT6 recognizing immune accessory cells by immunohistochemical methods. Ki-M6 binds to a lysosomal membrane-restricted antigen of 60,000 daltons without influencing significantly lysosome-related functions as far as the chemiluminescence response is concerned.