Abstract: The direct labeling method of monoclonal antibodies with generator produced rhenium 188 is studied in detail. Ascorbic acid is used as the reducer of antibodies and the protective agent against air oxidation as well. 188 ReO 4 - is reduced with SnCl 2 in the presence of citrate, which also acted as an intermediate complexing agent in the subsequent labeling step. The whole procedure is optimized and could be completed with 3 h. The final product possesses the radiochemical purity of higher than 95% with colloidal Re Less than 5%, and good stability in vitro as well as higher immunity.