The pore-forming protein, lipopolysaccharide-protein complex and lipopolysaccharide of Y. pseudotuberculosis outer membrane have been shown to participate in the penetration of the bacteria into the cells of the macroorganism, to produce a toxic effect on these cells and to enhance the ingesting activity of macrophages in small doses, while suppressing it in large doses. When introduced parenterally, protein induces a more pronounced clinical picture of specific reactive hepatitis in experimental animals and greater changes in their kidneys than the lipopolysaccharide--protein complex.