Two novel calcium-binding proteins which belong to the S100 protein family were isolated and sequenced. Using monospecific antisera their expression by myeloic/monocytic cells was shown. The two proteins may form complexes particularly a heterodimer which may also be expressed on the surface of infiltrating monocytes in acute inflammations. In vitro, its surface expression is induced by agents affecting the calcium household of cells. In contrast, formation of the heterodimer is conspicuously absent in chronic inflammatory lesions. In the latter situation monocytes either express MRP8 or MRP14 and not both as in acute inflammation. In all inflammation models tested so far the cells arriving first at the lesion were MRP8- and MRP14-positive. MRP8/14 which is identical with the cystic fibrosis antigen is also found in body fluids in inflammatory conditions and thus may be considered as a very sensitive inflammation marker. Soluble MRP8/14 complexes may exert different functions, e.g. inhibition of casein kinases, binding to cytoskeletal proteins, antimicrobial effects. MRP8 and MRP14 thus represent two novel molecular parameters of the early events of inflammatory reactions which reveal interesting aspects for the pathomechanism of chronic inflammatory reactions.