The transmembrane domain of the M2 protein from influenza A virus forms a nearly uniform and ideal helix in a liquid crystalline bilayer environment. The exposure of the hydrophilic backbone structure is minimized through uniform hydrogen bond geometry imposed by the low dielectric lipid environment. A high-resolution structure of the monomer backbone and a detailed description of its orientation with respect to the bilayer were achieved using orientational restraints from solid-state NMR. With this unique information, the tetrameric structure of this H(+) channel is constrained substantially. Features of numerous published models are discussed in light of the experimental structure of the monomer and derived features of the tetrameric bundle.