We have studied two anti-p185 antibodies: the monoclonal antibody 7. 16.4 and rhuMAb 4D5, which were raised against the the ectodomain of rat (p185(neu)), and the human (p185(her2/neu)) homolog, respectively. Studies on the structure of these two antibodies indicate that they share structural similarity in the variable region, especially the CDR3 region, which determines the antibody-antigen interaction. Further studies by flow cytometry revealed that 7.16.4 can compete with rhuMAb4D5 for binding to the cell surface p185(her2/neu), suggesting that these two antibodies share an epitope on the p185 receptor. Furthermore, 7.16.4 can also inhibit proliferation and transformation caused by p185(her2/neu). Moreover the rhuMAb 4D5 binds to the rat p185(neu). With the observation that 7.16.4 positively stains human breast cancer tissues that overexpress p185(her2/neu), 7.16.4 may be useful for the pathological diagnosis and therapy of human tumors.