During terminal maturation of blood monocytes (MO) into macrophages (MAC), a multitude of phenotypic and functional changes occur: cells increase in size and enhance their capacity for phagocytosis and tumor cytotoxicity, but decrease their ability for T-lymphocyte stimulation. The pattern of secreted cytokines is shifted as is the profile of surface antigens. The identity of the MAC maturation-associated antigen MAX.I/MAX.II with carboxypeptidase M (CPM), a phosphoinositol-linked endopeptidase, was recently described. CPM is able to process a multitude of different substrates, among them immunologically important peptides such as bradykinin, anaphylatoxins and enkephalins. It was previously shown to be expressed in placenta, lung and kidney. CPM as detected by MAX.I/II shows a strong expression on MO-derived MAC in vitro and on MAC in vivo accompanying T-lymphocyte activation such as during allogeneic transplant rejection or allergic alveolitis. In contrast, its expression is suppressed on MAC by some types of tumor cells. A synchronous expression of CPM together with MAC cytotoxic function makes a functional relationship very well possible. However, the biological importance of CPM expression on MAC in vivo is difficult to predict, since a wide range of biologically active peptides are substrates for CPM, and the relevance for most of those peptides to be processed by CPM during an immune reaction is only poorly understood at present.