We demonstrate a novel bone degradation pathway in primary bone cancer. This mechanism combines MMP-dependent proteolysis and collagen endocytosis and is mediated directly by mesenchymal tumor cells. It complements the well-established osteoclast-mediated degradation mechanism that is dominant in secondary bone cancers of epithelial origin. Immunohistochemical examination of sections from human osteosarcoma revealed a dominant expression of the collagenolytic metalloprotease MT1-MMP on tumor cells, along with the endocytic collagen receptor, uPARAP/Endo180. These components take part in a combined pathway of collagenolysis and are involved in bone matrix turnover during normal bone growth. In osteolytic lesions of primary bone cancer, osteosarcoma cells positive for these two proteins formed a contiguous layer aligned with the bone degradation zones where, strikingly, osteoclasts were scarce or absent. To test the functional importance of these components in osteosarcoma-mediated bone destruction, we utilized an osteolytic bone tumor model in mice for therapy experiments with a mouse monoclonal antibody against murine uPARAP/Endo180. This antibody prevented collagen turnover by mouse sarcoma cells in vitro and depleted transplanted bone tumors for uPARAP/Endo180 in vivo. When tumor-inoculated mice were subjected to systemic treatment with this antibody, a strong reduction of bone destruction was obtained. Our findings demonstrate the importance of MMPs and collagen endocytosis in primary bone cancer. Furthermore they point to uPARAP/Endo180 as a novel target to improve treatment of tumor-mediated bone degeneration, being relevant both in connection with neoadjuvant therapy and as a treatment in the case of inoperable tumors.