Abstract: 198Au has good nuclear property for application to the labelling of monoclonal antibody, eventually to the diagnosis and the therapy of tumours. An effective method is found to label antibody with Au by using reducing agents such as NaBH4 ,NaHSO3 to reduce disulfide in the protein into free thiols with which Au(Ⅰ) could be bound. By this method 199Au-labelled horse polyclonal antibody against human IgG without loss of its immunoreactivity is prepared successfully. This method could also be employed to label monoclonal antibody.