Samples of human serum γ-globulin with specifically bound copper or zinc cations were studied in Mancini’s radial immunodiffusion test with human antibodies to IgG (H+L). The intensity of antibody binding to zinc-modified protein was 10–20% higher in comparison with the reference sample, while detection of γ-globulin with bound copper by antibodies was 20–30% lower than in the corresponding reference sample. Comparison with the results of native γ-globulin testing indicates limitations of Mancini’s method as the quantitative assay for practical diagnosis, because under certain clinical conditions the traditional method can give over-and underestimated results.