Two-dimensional proton nuclear magnetic resonance (n.m.r.) experiments were performed on the coat protein of cowpea chlorotic mottle virus (molecular mass: 20.2 kDa) present as dimer (pH 7.5) or as capsid consisting of 180 protein monomers (pH 5.0). The spectra of both dimers and capsids showed resonances originating from the flexible N-terminal region of the protein. The complete resonance assignment of a synthetic pentacosapeptide representing this N terminus made it possible to interpret the spectra in detail. The capsid spectrum showed backbone amide proton resonances arising from the first eight residues having a flexible random coil conformation, and side-chain resonances arising from the first 25 N-terminal amino acids. The dimer spectrum showed also side-chain resonances of residues 26 to 33, which are flexible in the dimer but immobilized in the capsid. The n.m.r. experiments indicated that the conformation of the first 25 amino acids of the protein in dimers and capsids is comparable to the conformation of the synthetic peptide, which alternates among extended and helical conformations on the n.m.r. time-scale. It is suggested that the alpha-helical region, situated in the region between residues 10 and 20, binds to the RNA during assembly of the virus particle.