Abstract Psophocarpin B 1, one of the major seed storage proteins of Psophocarpus tetragonolobus (winged bean), has been purified to homogeneity and crystallized. It is an acidic protein with a pI of 5.5 and M r of 20 000, as determined by gel filtration in the absence of denaturating agents. Polyacrylaniide gel electrophoresis in the presence of sodium dodecyl sulphate (SDS-PAGE), indicates the absence of a quaternary structure of the protein. Using specific antibody raised in rabbits, a radio-immunoassay (RIA) method has been developed for its quantitation. It shows that 12% of the total soluble protein in the tuber of this plant is psophocarpin B 1 whereas it represents 9% of the seed protein. In the pods, it is only a minor component (1% of the total soluble protein) and it is not detectable in stems and leaves. Psophocarpin B 1 undergoes a time-dependent degradation in germinating seeds.