It was shown with the aid of immunosorption of an allergen-active substrate of E. coli 020: K84 (No. 2-rII) that protein substances taking part in the phenomenon of cell hypersensitivity were active in the humoral immunity reactions. The allergenic and immunochemical activity served as functions of the same molecules of bacterial proteins, this substantiating the use of immunochemical analysis for the study of an allergen-active bacterial substrate. By protein denaturing it is possible to obtain immunochemical inert allergen-active preparations capable of detecting the cell hypersensitivity to crude bacterial proteins. The problem of immunological polyfunctionality of proteins is discussed from the aspect of nonhomogeneity of their antigenic determinant groups.