To obtain a composition capable of being used for preparing a protein- like agent useful for radioactively labeling a protein including an antibody with a radioactive nuclide by adding a reducing protein having a specific reactive group and an Sn(II)-reducing agent in a suitable concentration.

This composition contains (A) a reduced protein (preferably a reduced monoclonal antibody, its fragment or a reduced polyclonal antibody) having one or more reactive groups which can form a complex with Sn(II), and (B) an Sn(II)-reducing agent 111 a concentration which can perfectly reduce an added radioactive nuclide and which does not accelerate the further reduction of the component A. The composition preferably further contains a carrier, an inert sugar or a non-reduced protein. The composition is useful for preparing proteins having the stable labels of the radioactive nuclide of technetium or rhenium.