TP3-pokeweed antiviral protein (PAP) immunotoxin is directed against the p80 antigen on osteosarcoma cells. Previous studies have demonstrated that TP3-PAP kills clonogenic human osteosarcoma cells in vitro and shows significant antitumor activity in a murine soft tissue sarcoma model (P. M. Anderson, et al, Cancer Res., 55: 1321-1327, 1995.) In this study, we demonstrate that TP3-PAP elicits p...