X-ray diffraction, density measurements, and stereochemical data were used in order to disclose the architecture of murein, the rigid component of almost all bacterial cell walls. Dry densities of 1.38 -1.39 g/cm3 were observed for Micrococcus luteus and Staphylococcus aureus. The X-ray data for gram-positive (S. uureiis, M . luteus) and gram-negative (Escherichia cofi) strains were almost identical, showing, in addition to some diffuse scattering with broad maxima corresponding to Bragg values of 0.22 and 0.45 nm, two sharp peaks indicating periodicities of about 0.7 and 0.94 nm. These latter periodicities were not found in whole bacteria but emerge immediately after breakage of the whole cells. In gram-positive species a weak oriented reflex at 4.2 nm appeared, while all other reflexes remain ring-shaped. Diffraction patterns obtained under the influence of humidity and certain metal ions showed strong variations in the position of the 0.45-nm ha1 0.These data, together with stereochemical considerations, invalidate the hitherto advanced models of a chitin-like structure of the glycan chains in murein. Instead, the following model proposed and discussed here is consistent with the experimental data. The structure is made up of layers, which in S. aureus and M . luteus are separated by about 4.2 nm. The layers are more or less randomly rotated against each other. The sugar chains run parallel to these layers and, hence, to the surface of the cell wall. They do not possess a twofold screw axis as in chitin or cellulose. Thus, the peptide strands protude from the glycan chain axis in different directions. The peptide strands seem to assume fairly rigid conformations and to be mainly responsible for the regular layer-like arrangement of murein.The main skeletal component of most bacterial cell walls is a unique polymer called peptidoglycan (murein). The murein of gram-positive bacteria has a multilayered arrangement and makes up about 50 % of the cell wall weight, whereas the murein of gramnegative bacteria is probably monolayered, representing only about 5-10% of the cell wall weight [l].The glycan chains are composed of alternating sequences of N-acetylglucosamine and N-acetylmuramic acid. The peptide part is built up by alternating Dand L-amino acids, including a y-bond for D-glutamic acid (Fig. 1) [2-41.The subunits are linked together by p(1-4)-glycosidic bonds in the glycan part and can be crossParts of this paper were presented at the IV. International Conference on Small-Angle Scattering of X-Rays and Neutrons in Gatlinburg/U.S. A., October 1977. linked by peptide bonds between the free amino group of the diamino acid and the carboxyl group of the D-alanine of another peptide strand which, in this case, loses its second D-alanine. The cross-linkage COCH..