Abstract: Objective To investigate the inhibitory effects of 188Re-labeled herceptin on the proliferation in vitro of breast carcinoma cell line (SKBR-3) overexpressing HER-2/neu proto-oncogene. Methods Herceptin was radiolabeled with 188Re through a direct labeling method. SKBR-3 cells were cultured with 188Re-Herceptin at different radioactivity doses (3.7×104 , 18.5×104, 37×104, 55.5×104 and 74×104 Bq/ml) or with 188Re-nmIgG and 188ReO4- for comparison. The cell proliferation inhibition was determined with MTT colorimetric assay. Results 188Re-Herceptin could markedly inhibit the growth of SKBR-3 cells in a radioactivity dose-dependent fashion, while the effect of 188Re-nmIgG and 188ReO4- showed rather poor inhibitory effect in vitro. The 50% inhibition doses (IC50) of 188Re-Herceptin, 188Re-nmIgG and 188ReO4- were 76.1×104 Bq/L, 139.2×104 Bq/L and 175×104 Bq/L, respectively. Conclusion 188Re-Herceptin can effectively inhibit the growth of in vitro cultured breast cancer cells overexpressing HER-2/neu, and shows much potential for clinical use in beast cancer radioimmunotherapy.