[en] Peptidoglycan, neutral polysaccharide, and anionic polysaccharide are the main structural components of the walls of Lactobacillus acidophilus strain 63 AM Gasser. The peptide moiety of the peptidoglycan is essentially composed of Nα-(L-alanyl-D-isoglutaminyl)-L-lysyl-D-alanine peptides, cross-linked by Nє-(D-alanyl-D-isoasparaginyl)-L-lysine bridges. The extent of peptide cross-linking is low. In log phase walls, the average size is of 2.3 cross-linked peptides, 10% of the peptide subunits occurring as monomers, 37% as dimers, and 30% as trimers. All of the peptide monomers and oligomers have D-alanyl-D-alanine sequences at their C termini, identified and measured with the help of a Streptomyces DD carboxypeptidase. The glycan moiety of the peptidoglycan is composed of β-1,4-N-acetylglucosaminyl-N-acetylmuramic acid units. All of the muramic acid residues are peptide substituted and about 60 to 70% of them have O-acetyl substituents on C6. The neutral polysaccharide is composed of equimolar amounts of glucose, galactose, and rhamnose. It is apparently linked to muramic acid residues of the peptidoglycan via phosphodiester groups. The extent of bridging is about 1 phosphate group for (glucose70-rhamnose70-galactose70) units. The anionic polysaccharide is a mixture of (a or β)-1,6-linked polyglucose polymers with monomeric α-glycerol phosphate side chains attached to them on C2 or C4 position. Peptidoglycan, neutral polysaccharide, and anionic polysaccharide represent 42, 32, and 17%, respectively, of the log-phase walls. Stationary-phase walls have similar composition and structure, except that substantial amounts of neutral polysaccharide are replaced by anionic polysaccharide.