The cell-wall peptidoglycan of Staphylococcus aureus is a heterogeneous, highly cross-linked polymer of unknown tertiary structure. We have partially characterized this structure by measuring spin diffusion from C-13 labels in pentaglycyl cross-linking segments to natural-abundance C-13 in the surrounding intact cell walls. The measurements were performed using a version of centerband-only detection of exchange (CODEX). The cell walls were isolated from S. aureus grown in media containing [1-C-13]glycine. The CODEX spin diffusion rates established that the pentaglycyl bridge of one peptidoglycan repeat unit of S. aureus is within 5 angstrom of the glycan chain of another repeat unit. This surprising proximity is interpreted in terms of a model for the peptidoglycan lattice in which all peptide stems in a plane perpendicular to the glycan mainchain are parallel to one another.