Escherichia coli murein, the polymer from which the shape-maintaining structure of the cell envelope is made, shows unexpected complexity. The separation of murein building blocks with high performance liquid chromatography reveals about 80 different types of muropeptides. Their behavior in high performance liquid chromatography and their chemical structure are described. The complexity of E. coli murein is due to the free combination of seven different types of side chains (L-Ala-D-Glu-R with R = -OH, -m-A2pm, -m-A2pm-D-Ala, -m-A2 pm-Gly, -m-A2pm-D-Ala-D-Ala, -m-A2pm-D-Ala-Gly, -m-A2pm-Lys-Arg) with two types of cross-bridges (D-Ala-m-A2pm, -m-A2pm-m-A2pm). The novel type of cross-bridge, A2pm-A2pm, contains an L,D-peptide bond, as shown by Edman degradation and chemical analysis of the reaction products. The A2pm-A2pm cross-bridge is assumed to play a role in the adaptation of the cross-linkage of murein to different growth conditions of the cell. The structural data of E. coli murein agree best with a model of a thin, however multilayered, murein sacculus.