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A new method permits direct densitometric quantitation of different molecular-size components obtained by thin-layer Sephadex gel filtration. A transparent plate suitable for protein staining and transmission densitometry is prepared by over-layering agarose after the filtration. Analysis of commercial γ-globulin preparations with this method gave better resolving capacity than ultracentrifugation of column gel filtration. Furthermore, this method is easily adapted by means of immunogel filtration to specific characterization of the molecular size categories. As many as nine IgG-related components differing in size were demonstrated. Component categories containing as little as 0.06 µg could be detected. The error of the method varied between 1.9% and 13% for the different size categories. Good reproducibility was observed when the method was used for analysis of preparations with known qualitative and quantitative IgG differences. [ABSTRACT FROM AUTHOR]