Calprotectin, also called MRP 8/MRP 14, is a complex of calcium binding proteins which belong to S-100 protein family. Calprotectin is selectively present in myeloid cells, namely, neutrophils or macrophages. It is especially abundant in cytosol of the former cell type. It has been observed that protein complex highly increases in extracellular fluids of patients with many inflammatory diseases. We recently elucidated that calprotectin induced apoptosis of tumor cell lines as well as normal fibroblasts, suggesting that the factor modulates inflammatory reaction through the effect on survival or growth states of fibroblasts or other cell types. It is also possible that high amount of calprotectin in inflammatory sites may cause tissue destruction. Several authors also reported the extracellular functions of the factor, including anti fungal activity, chemotactic activity and activity modifying leucocyte functions. The reports accumulated recently, together with ours, raise the possibility that calprotectin may be not only an useful marker but also an important mediator in inflammatory reactions.