The peptide alpha Ahx-Met-Ala-Asp-Pro-Asn-Arg-Phe-Arg-Gly-Lys-Asp-Leu-Pro-Val-Leu- Asp-Gln-Leu-Thr-Asp-Pro-Pro-alpha Ahx (epsilon Ahx = 6-aminohexanoyl), the antigenic sequence 11-32 from Herpes simplex virus glycoprotein D-1, has been synthesised. Its 1H-NMR spectrum has been assigned by a combination of two-dimensional techniques in H2O and 2H2O. Its secondary structure has been defined by nuclear Overhauser effects and amide proton exchange rates, and also to some extent chemical shifts, coupling constants and amide proton temperature coefficients. These latter parameters are shown to be less reliable as guides to secondary structure. The peptide has a helical (type I/III) turn at residues Pro-14-Asn-15 and helical structure at residues Lys-20-Val-24, in rapid equilibrium with random-coil structure. A beta-turn at residues Arg-18-Gly-19 may be present as a minor component. These locations of secondary structure correspond with previously determined regions of antigenic activity.