Genetic studies with the crosses and backcrosses of SWR/J and A/J mice indicate that the idiotypic determinants of Group A streptococcal antibodies segregate in a Mendelian fashion. The idiotypic reactions were inhibited with phenyl β N acetyl glucosaminide which contains immunodominant determinants of the Group A carbohydrate. Some of the absorbed idiotypic antisera to SWR/J and A/J Group A antibodies failed to detect strain specific idiotypic determinants. Such anti idiotypic sera exhibited interstrain idiotypic cross reactions with the Group A antibodies from a number of different mouse strains. These idiotypic cross reactions were also inhibited by phenyl β N acetyl glucosaminide, a finding which suggests that these interstrain idiotypic cross reactions are dependent at least in part on site associated determinants.