The clinico-biochemical examination of 125 patients with different forms of diphtheria was carried out to improve the quality of diagnostic of diphtherial infection. It included the complex investigation of readings of glycoproteins and isofermentative spectrum of aminotransferases. There was established that in the development of pathological process in diphtherial infection the disturbance of metabolism in connective tissue play an important role and isofermentative spectrum of aminotransferases is characterized by the marked disbalance mainly with the increase of mitochondrial isoferments. The degree of the revealed changes is clearly correlated with the severity of diseases and pathological shifts in toxic form of diseases are preserving after the end of the acute phase of diseases as diphtherial complications.