Recently we cloned a novel receptor tyrosine kinase, STK. STK belongs to the hepatocyte growth factor receptor family and was identified as the receptor for macrophage-stimulating protein (MSP). STK is expressed on a restricted, macrophage population such as peritoneal macrophages, but not on mononuclear phagocytes of peripheral blood, bone marrow, or alveoli. Using an anti-STK monoclonal antibody, we observed STK expression on multinuclear osteoclast-like cells (OCLs) formed by murine bone marrow cultures in the presence of 1,25-dihydroxyvitamin D3, and interleukin-3. The OCLs expressed both the calcitonin receptor and STK. We also detected STK expression in bone-derived mouse osteoclasts. The addition of MSP to OCLs induced rapid morphologic changes such as cytoplasmic contraction and formation of ruffled border. In addition, MSP caused rapid redistribution of src to the borders of cytoplasm. These phenomena were associated with enhanced bone resorption. MSP caused a threefold increase in pit formation compared with control OCLs. These findings suggest that by involving src kinase, the MSP/STK signal transduction pathway induces rapid cytoskeletal reorganization in osteoclasts and facilitates bone resorption by osteoclasts.