Natural antibodies to glucosaminyl dipeptide (GMDP), a component of bacterial cell-wall peptidoglycan, isolated from the serum of normal donors by thermal extraction, are capable of cross reaction with the determinant of the glycan chain: tetrasaccharide consisting of N-acetylglucosamine and N-acetylmuramic acid. The intensity of the interaction of natural anti-GMDP antibodies with a specific ligand is considerably higher than with tetrasaccharide. Natural antibodies to tetrasaccharide possess the properties of heterologous antibodies, i.e. the intensity of their interaction with a specific ligand (GMDP) is considerably higher than with a homologous ligand. Suggestion is made that GMDP is the specific antigenic determinant of peptidoglycan to which antibodies are formed in process of natural immunization. These antibodies react with different intensity with homologous (GMDP) and related (tetrasaccharide) haptens.