Abstract The immunochemical specificity of the common part of bacterial mucopeptide, which is composed of the polysaccharide backbone and the peptide side chains, was studied in a heterologous system consisting of soluble, nondialyzable, lysozymedigest of M. lysodeikticus cell walls as test antigen and anti-S. pyogenes group A or A variant antiserum. A modified S. aureus mucopeptide was used for inhibition studies. It was confirmed that the mucopeptide contains two antigenic determinants—the peptide and the polysaccharide. A more detailed study of the polysaccharide backbone specificity revealed that the immunodominant sugar is the β-N-acetylglucosamine, suggesting a terminal position for this amino sugar in the antigen. The experimental data favor the conclusion that mucopeptide was the main antigen in the heterologous precipitation, but constituted an insignificant part of the homologous precipitation.