Novel therapies that require internalization of effector domains may be improved by assessing the efficacy of postbinding receptor-mediated endocytosis. To achieve targeted gene therapy of immunotoxin therapy, natural vector-host tropisms must be altered. Recent improvements in monoclonal antibody (MAb) engineering have expanded the potential range of host cells that can be targeted for therapeutic intervention. However, relatively little is known about cellular responses after binding of a vector construct. We have tested the utility of four novel MAbs recognizing the extracellular domain of p185HER-2, a membrane receptor protein, for use in internalization-dependent therapies. All four antibodies bound to p185HER-2 in a number of immunoassays. Two antibodies recognized accessible epitopes of p185HER-2 on viable cells. Radioimmunoassay demonstrated that antibody-membrane receptor complexes formed by two antibodies were internalized and trafficked through an endolysosomal degradative pathway. Two of the four antibodies evaluated were found to have favorable internalization characteristics suitable for incorporation in a targeting vector. This analytical approach could be applied to antibodies prior to and after fusion with various vectors or toxins to determine the potential utility of the antibodies for targeted therapy.