Here, we report the successful use of an immunoPET probe based on 89Zr-radiolabeled, humanized anti-GPC3 monoclonal antibody (clone H3K3, obtained through complementarity determining region (CDR) grafting of the murine 1G12 clone) to accurately identify GPC3-positive HCC cells in vitro and in vivo. The gene discussed is GPC3; the disease is hepatocellular carcinoma.