The neu oncogene encodes a 185-kDa transmembrane glycoprotein tumor antigen, termed p185. We have recently described a monoclonal antibody reactive with a cell surface domain of the p185 molecule. In vivo treatment with this anti-p185 monoclonal antibody was able to significantly inhibit the tumorigenic growth of neu-transformed NIH 3T3 cells implanted into nude mice. Such treatment had no effect on the tumorigenic growth of Ha-ras-transformed NIH 3T3 cells. Furthermore, anti-p185 antibody treatment was able to inhibit the growth of the rat neuroblastoma cells from which the neu oncogene was initially isolated. These results demonstrate that a monoclonal antibody reactive with the extracellular domain of an oncogene-encoded protein can exert a significant antitumor effect; such antibodies may prove useful in the therapy of certain malignancies.