Using a monoclonal antibody to macrophage migration inhibition factor (MIF), two proteins were isolated from supernatants of Concanavalin A-stimulated human peripheral blood mononuclear cells which seem to have complexed to a third component carrying the MIF activity. They are therefore designated MIF-related proteins or MRP-8 and MRP-14 according to their apparent molecular weights. Partial amino acid sequences have been determined and their cDNA have been cloned and expressed in Escherichia coli. Both are calcium-binding proteins and MRP-8 seems to be largely homologous to the cystic fibrosis antigen (Dorin et al., 1987). Antisera were raised in the rabbit against the recombinant proteins and their expression in cells and tissues studied using immunohistological techniques. The proteins are only found in blood granulocytes and monocytes. In culture the number of positive monocytes sharply increased and then declined with time, suggesting that their expression is associated with early stages of monocyte/macrophage differentiation and absent from resident macrophages in all tested tissues. In acute inflammatory reactions, e.g. gingivitis, MRP-8 is never seen in the tissue, whereas MRP-14 is expressed by intravascular monocytes and perivascular macrophages. In contrast, in chronic inflammation, e.g. rheumatoid arthritis, MRP-8 is also expressed by macrophages in the tissue. From this it is concluded that MRP-8 and MRP-14 are expressed sequentially at defined stages of monocyte/macrophage differentiation and that dysregulation of this process in chronic inflammation is mirrored by the presence of MRP-8-positive macrophages in the tissue.