Limited hydrolysis of EF-2 with trypsin in mild conditions leads to cleavage at the N-terminal part of the protein, at the region of phosphorylation, at the Arg54 and Arg65 residues. The trypsinolysis product, fragment T1', containing Thr56 and Thr58, which are phosphorylated in EF-2, is also phosphorylated by EF-2-kinase at the same residues. In the phosphorylated EF-2, digestion by trypsin takes place only at Arg65, resulting in a reduction of the rate of hydrolysis in comparison with the native EF-2. Digestion of EF-2 with elastase results in the formation of two fragments E1 and E2 (60 and 40 kDa, respectively). Fragment E1 represents the N-terminal part of EF-2. It is resistant to the further action of elastase, is not cleaved by trypsin, and loses its capability for phosphorylation. Fragment E2, the C-end part of the molecule, is not resistant to the further action of elastase and retains its capability for ADP-ribosylation with the A fragment of diptheria toxin and NAD+. Electrophoretic analysis of EF-2 and its proteolytic fragments according to O'Farrell showed that the modification, resulting in the presence of two initial forms of EF-2, is located between the amino acid residues 66 and 506 of the polypeptide chain. In conclusion a possibility of studying the formation of partial functional activities within the framework of individual structure-functional domains using a set of N-terminal fragments of various length is discussed.