An insoluble macromolecular Sn(II)(R-Sn) complex which strongly binds Sn(II) by chelation was applied to the direct 9 9 m Tc labeling of human immunoglobulin(IgG) to minimize the influence of Sn(II). 9 9 m Tc labeling was achieved at greater than 90% yield simply by the short-term mixing of IgGa containing >2 -SH groups per IgG molecule, 9 9 m Tc pertechnetate, and the R-Sn complex in pH 7 solution. The 9 9 m Tc-IgGa obtained by this labeling method has high stability based on the thiol-specific binding of 9 9 m Tc without transchelation from another weakly bound 9 9 m Tc-complex.