Degradation of heat-treated cells of Proteus vulgaris P 18 with a protease preparation from a soil Bacillus subtilis allowed the isolation of a cell envelope structural entity, 60-70 Å thick, consisting of two electron-dense layers separated by a light one and composed, at least in part, of lipopolysaccharide O-antigen and of peptidoglycan. An enriched preparation, of which 55 % of the dry weight consisted of an intact peptidoglycan, was obtained through the sequential action upon lyophilized cells of hot aqueous sodium dodecyl sulfate and B. subtilis protease. Both Chalaropsis B endo-N-acetylmuramidase and Streptomyces DD carboxy-peptidase solubilized the enriched peptidoglycan preparation. The Chalaropsis enzyme completely degraded the glycan moiety into peptide-substituted disaccharide units. The DD carboxypeptidase hydrolyzed the D-alanyl-(D)-meso-diaminopimelic acid linkages involved in peptide cross-linking. In the intact peptidoglycan, about 35 % of the L-alanyl-γ-D-glutamyl-(L)-meso-diaminopimelic acid-(L)-D-alanine residues occurred as uncross-linked monomers, 40 % of them as peptide dimers and 16 % as peptide trimers. It is likely that not all the peptides retained the D-alanine residue at their C-termini. About half of the N-acetylmuramic acid residues in the glycan moiety are O-acetylated, a property which is compatible with the high lysozyme resistance exhibited by the P. vulgaris peptidoglycan.