Osteosarcoma is the commonest malignant tumour of the bones. The presence of micrometastases at the time of primary diagnosis is associated with poor prognosis. Despite developments in surgery and aggressive chemotherapy, about 50% of the patients still succumb to the disease. Thus, there is a need to develop alternative treatment modalities. One such strategy is to use antibodies with improved effector functions. The two monoclonal antibodies, TP-1 and TP-3, recognize a tumour-associated antigen on human osteosarcoma cells. In the present study, we describe the cloning of the TP-1 variable genes, and the production of complete chimeric mouse/human monoclonal antibodies. Constructs containing the constant genes from human IgG1, IgG3 or a mutant IgG3 with a shortened hinge region, called m15, were expressed in the mouse myeloma cell line, NS0. The m15 mutant has been shown to be very potent in triggering complement-mediated lysis. Our goal was to investigate whether this mutant could overcome the complement protection on human osteosarcoma cells, which is generally present on all human cells. We found that the target cells expressed several membrane-bound complement inhibitors, and that masking of these inhibitors rendered the cells sensitive to lysis. The m15 mutant exhibited greater lytic activity than both IgG3 and IgG1, although it could not cause extensive killing of the target cells alone.