The comparative study of two methods of the proteolysis of IgG with the aim of obtaining F(Ab)2-fragments from the blood sera of patients and healthy donors has been made. Different methods for the isolation of F (ab)2-fragments, such as gel filtration on Sephadex G-200, affinity chromatography on protein A-BrCN-sepharose, reprecipitation with zinc sulfate, have been analyzed. For further work the method of the peptic decomposition of whole serum with subsequent salting out with ammonium sulfate and gel filtration on Sephadex G-200 has been chosen.