As antigen-presenting and/or monokine-secreting cells, macrophages play a major role in immunoregulation. Proteases of macrophage origin (cathepsin G, elestase, thrypsin and pronase) act on cell surfaces of different cell lines, inducing cell activation, e.g. of B-lymphocytes. T-lymphocytes might be stimulated by the activating factor LAF. Other macrophage products (CSF, FIM) control monocyte production in bone marrow. While lymphocytes are the target cell lines for classical immunosuppressive agents, mononuclear phagocytes are kept for the major cell population affected by antiinflammatory drugs. The presented study outlines the significance of the mononuclear-phagocytesystem in antiinflammatory drug research. The inhibiting potency of antiinflammatory drugs on the monocyte-macrophage cell line as an additional immunoregulatory principle should be discussed.