The data are presented on the effect of activation of by peptidoglycans from cell walls of Staphylococcus aureus and Brevibacterium flavum on the DNA synthesis intensity in the mice spleen, thymus and bone marrow in vivo. Maximum of the peptidoglycan influence on the DNA synthesis intensity is observed 12-36 hours after injection. As a rule, higher doses of the studied peptidoglycans (1 mg per one animal) activate the DNA synthesis more efficiently. Peptidoglycan from Br. flavum stimulates the DNA synthesis in all tissues more intensively then the peptidoglycan from S. aureus, which may be a result of its monomers' structure.