The antibody response of rabbits to Micrococcus lysodeikticus is characterized by the production of a high concentration of antibodies which manifest markedly reduced heterogenicity. The specificity of these antibodies was studied and it revealed that M. lysodeikticus contains 2 major antigens: both the glucose-N-acetyl-aminomannuronic acid polymer obtained by formamide extraction of the cell walls and peptidoglycan solubilized by ultrasonic treatment gave precipitin reactions with hyperimmune antisera. By means of inhibition studies of the glucose-mannose polymer specificity, glucose appeared as the immunodominant sugar in the majority of antibodies studied. Inhibitions studies also confirmed that both the glycan and peptide moieties constitute antigenic determinants of M. lysodeikticus peptidoglycan. Antibodies to the glucose-mannose-polymer and the peptidoglycan were specifically fractionated by use of immunoadsorbents formed from lysozyme solubilized cell walls and activated Sepharose. Both antibody specificities showed a limited heterogeneity by isoelectric focusing. Finally, because antisera to M. lysodeikticus are a rich source of antibodies to peptidoglycan, emphasis is placed on the possible usefulness of this system for studies of clonal dominance.