The determination of the three-dimensional solution structure of alpha1-purothionin using a combination of metric matrix distance geometry and restrained molecular dynamics calculations based on n.m.r. data is presented. The experimental data comprise complete sequence-specific proton resonance assignments, a set of 310 approximate interproton distance restraints derived from nuclear Overhauser effects, 27 Ø backbone torsion angle restraints derived from vicinal coupling constants, 4 distance restraints from hydrogen bonds and 12 distance restraints from disulphide bridges. The average atomic rms difference between the final nine converged structures and the mean structure obtained by averaging their coordinates is 1.5 +/- 0.1 å for the backbone atoms and 2.0 +/- 0.1 å for all atoms. The overall shape of alpha1-purothionin is that of the capital letter L, similar to that of crambin, with the longer arm comprising two approximately parallel alpha-helices and the shorter arm a strand and a mini anti-parallel beta sheet.