Many bacteria, including Streptococcus pyogenes are able to suppress the development of tumors as at humans as at other mammals; however, mechanisms of the antitumor activity of bacteria are still unknown. Data about the role of the arginine deiminaze of streptococci as an important antitumor agent have been published. Purpose: in the present study the role of the arginine deiminaze in the antitumor activity of bacteria has been studied. Materials and methods: in course of performed studies the influence on the progress of tumors for cultures of two strains of Streptococcus pyogenes with the serotype M49 was compared: the M49-16 and M49-16delAD strains that produce the arginine deiminaze and that don’t produce it. In the first group of experiments performed in vitro, the culture of cells of hepatoma of 22a mice was affected by streptococci. Results: by means of the xCELLigence system (ACEA, USA), that makes possible to evaluate the condition of the culture of the tissue on a real-time basis it was shown that only streptococci that produce the arginine deiminaze have the cellulotoxic activity. During the second group of experiments performed on laboratory animals suffering with M37 sarcoma it was determined that the administration of streptococci that produce the arginine deiminaze into the tumor caused the decrease of the dimension of the tumor and as well as the decrease of the mortality of animals. The administration of streptococci that don’t produce this ferment did not have such effect.