Bone resorption resulting from the metastatic human melanoma cell line (A375) was investigated morphologically using an experimental model of bone metastases in nude mice. An injection of A375 (1 x 10(5)) in the left ventricle produced multiple osteolytic lesions. Many TRAPase-positive multinucleated cells, identified by EM as osteoclasts, were observed on the bone surface at the site of metastases. The findings suggest that bone resorption was caused by osteoclasts developed in the presence of tumor cells. Even where tumor cells were juxtaposed to bone surface, small and flat TRAPase-positive cells were shown to exist on the bone surface. Thus, bone resorption was mainly associated with the occurrence of osteoclasts. A large number of osteoclast progenitor cells were also observed adjacent to tumor cells and/or stromal cells located apart from bone, indicating possible participation of tumor cells and/or stromal cells in the differentiation of osteoclasts. Ultrastructurally, stromal cells and/or extracellular matrices were present between tumor cells and osteoclast progenitor cells. Immunohistochemical observation clarified the localization of heparan sulfate proteoglycan (HSPG) and fibronectin (FN) around osteoclast progenitor cells. These findings suggest that they play an important role in providing a microenvironment favorable for osteoclast differentiation and activation. The immunohistochemical localization of IL-6, PGE2, and TGF-alpha also indicates that they are involved in osteoclast differentiation and activation. In conclusion, bone resorption at the metastatic sites of A375 is mediated via osteoclasts and A375 cells may be involved in the differentiation and activation of osteoclasts in association with stromal cells, extracellular matrices (HSPG, FN) and osteotropic cytokines (IL-6, PGE2, TGF-alpha).