We generated three anti-ErbB-2 monoclonal antibodies (mAbs) by using surface-epitope masking method (SEM method). They were characterized and shown to recognize the extracellular domain (ECD) of ErbB-2 specifically. In order to detect ErbB-2 in the sera of breast cancer patients, we used two of them: A18 and A21 to develop a sandwich enzyme-linked immunosorbent assay (ELISA). Sera from 10 of 57 patients (18%) and two of 142 controls (1.4%) had soluble ErbB-2 values higher than the cut-off level (30 ng/ml). The number of cancer patients with elevated sera ErbB-2 levels was significantly greater than that of the control group (P<0.005). Besides, three of the four cancer patients with metastasis (75%) had elevated serum ErbB-2 levels. We also purified ErbB-2 by A18-conjugated immunoaffinity chromatography and used it as the standard in the sandwich ELISA. Based on our preliminary clinical data, we came to the conclusion that both A18 and A21 could possibly be used in the prognosis of breast cancer.