The antitumor effect of peptidoglycans of various structures extracted from different gram-positive bacteria was studied, on chemically induced fibrosarcomas, in C3H/He, C57BL/6, and (C57BL/6 X C3H/He)F1 mice. When given sc admixed with tumor cells, only some peptidoglycans (those extracted from Bacillus megaterium and Staphylococcus aureus) enhanced tumor resistance in syngeneic and semiallogeneic hosts, whereas other peptidoglycans (those extracted from Micrococcus lysodeikticus and Corynebacterium poinsettiae) possessed no antitumor effect. When tumor cells were given ip, administration of peptidoglycans by the same route was either without effect on tumor growth or it induced tumor enhancement. Enhancement could be observed with all of the peptidoglycans tested. The antitumor effect when given sc and the ability to stimulate the proliferation of B-lymphocytes were shared by the same two peptidoglycans, while the other two peptidoglycans were devoid of both activities. It appears that these biological activities depend on the structure of the peptide moiety of peptidoglycans and that mitogenic and antitumor responses are stimulated by similar structures.