The terminal step in the maturation of mononuclear cells from circulating monocytes to resident macrophages is accompanied by dramatic changes in cell morphology and physiology. Applying a cultivation system which allows peripheral monocytes to undergo terminal maturation in vitro under absolutely endotoxin-free conditions, we have determined the pattern of expression of a set of eight genes by mRNA phenotyping. The results can be summarized as follows: the two protease inhibitors al-antitrypsin and a2-macroglobulin show a inverse pattern of expression. al-Antitrypsin mRNA is repressed, a,-macroglobulin mRNA is strongly induced during maturation to macrophages. Therefore, these two genes are excellent markers of the terminal maturation. In addition, ferritinlight-chain mRNA progressively increases during the course of differentiation, providing a further marker for maturation. Gene expression as a function of activation was studied in mononuclear cells stimulated with bacterial endotoxin (lipopolysaccharide). In monocytes, complement-factor-B, interleukin-1 and interleukin-6 mRNAs are drastically induced upon lipopolysaccharide activation whereas lysozyme RNA is strongly repressed. However, the ability of all four genes to respond to endotoxin was markedly diminished or abolished in mature macrophages, indicating that susceptibility to a certain type of activation may be restricted to a specific stage of maturation. Our data show that mRNA phenotyping is excellently suited for the characterization of the differentiation and activation state of mononuclear phagocytes.The mononuclear phagocyte lineage consists of cells at different stages of maturation linking monoblasts and promonocytes as precursors in the bone marrow with circulating blood monocytes, which leave the bloodstream and become resident macrophages in a variety of tissues and body cavities (for reviews, see [l -31). Besides ingestion and killing of bacteria and parasites and in addition to serving as antigenpresenting cells to lymphocytes, mononuclear phagocytes function upon injury or infection to release a wide range of cytokines acting as inflammatory mediators. Recently mononuclear phagocytes were found to also secrete factors central to the wound-healing response [4].Cells of the mononuclear phagocyte system exhibit marked heterogeneity. Differentiated macrophages can be distinguished from monocytes not only by increased size and maturation of organelles [5 -91 but also by substantial changes in the pattern of surface antigens [9 -171. The situation is complicated, however, in that both monocytes and macrophages change their physiological behaviour upon stimulation by bacterial agents such as the endotoxin lipopolysaccharide [18]. It is therefore desirable to establish a marker system which allows characterization of mononuclear phagocytes in regard to the stage of maturation and the state of activation.We have employed a previously described in vitro cultivation system [15,19, 201 Abbreviations. IL-6, interleukin-6; IL-1, interleukin-1.monocy...