<jats:p>Castanogenin, the major genin produced on hydrolysis of the saponin of Castanospermum australe Cunn, et Fras. is shown to be identical with medicagenic acid (2β,3β-dihydroxyolean-12-en-23,28-dioic acid). Further proof of several of the structural features of medicagenic acid is given. The minor sapogenin, bayogenin, C30H48O5, is shown to be 2β,3β,23-trihydroxyolean-12-en-28-oic acid by conversion into the same tetrol as is formed from medicagenic acid and by the formation of a γ-lactone by interaction of the carboxyl function and the 12,13-double bond.