A humanized monoclonal antibody directed against the cell surface oncofetal protein glypican-3 (GPC3) with potential antineoplastic activity. Anti-GPC3 monoclonal antibody GC33 binds to GPC3 and triggers a host immune response against GPC3-expressing tumor cells, which may result in tumor cell death. GPC3, a heparin sulfate proteoglycan, is frequently upregulated in hepatocellular carcinoma and mesoderm-derived organs such as the liver, lungs, and kidney.