A simple and generally applicable method for labeling antibodies with technetium-99m (99mTc) is described. Following reduction of intrinsic disulphide bonds, the antibody is labeled with 99mTc in the presence of a weak competing ligand methylene diphosphonate. High labeling efficiencies (greater than 97%), in a final labeling step taking only a few minutes, can be routinely obtained with high in-vitro stability over 24 hr. No effect upon antibody reactivity is seen.