[en] α,α'-Diaminopimelic acid (DAP) occurs in the wall peptidoglycan of Bacillus megaterium KM predominantly in the form of its meso isomer (about 85% of the total residues) and, in minor amounts, in the form of its DD isomer. The amino groups on the L carbon of the meso-DAP residues are involved in peptide linkages to the glutamic acid residues. Most of the amino groups on the D carbon of the meso-DAP residues are free; some of them are substituted, thus probably serving to cross-link peptide subunits. These amino groups can be liberated by a Streptomyces endopeptidase. None of the DD-DAP residues have amino groups free. Moreover, these groups are not liberated by endopeptidase treatment. The peptidoglycan upon enzymatic degradation yields mainly two fractions. A major fraction is composed of disaccharide peptide monomer subunits containing only the meso isomer of DAP. A second minor fraction is composed of disaccharide peptide oligomers containing both meso and DD isomers of DAP. The meso-DAP residues isolated as monodinitrophenyl derivatives from both fractions have optical rotations and optical rotatory dispersions identical with that of synthetic monodinitrophenyl-meso-DAP obtained by dinitrophenylation of the amino group on the D carbon. The assignment of the DD configuration to the DAP residues which are not meso rests upon the optical rotatory properties of their bisdinitrophenyl derivatives.