In an investigation of some Australian woods for saponin content, it was noted that Castanospermum australe gave strongly positive tests, and the present work was undertaken to investigate the saponin present in the wood. Extraction of the wood gave fats, phenols, sugars, and sapogenins. It was difficult to obtain crystalline material from the crude sapogenin; the method eventually adopted was to carry out a fractionation by long extraction with ether, whereafter the ether-soluble portion could be crystallized from methanol and resolved by fractional crystallization from methanol into a more soluble sapogenin, castanogenin, and a less soluble one not yet investigated. Analyses of castanogenin and its derivatives are consistent with the molecular formula C28H42O6 for castanogenin, and indicate the presence of two carboxyl and two hydroxyl groups. With tetranitromethane castanogenin itself gives no colour; its more soluble derivatives, the diacetate and dimethyl ester, however, give deep yellow colours, indicating the presence of a carbon-carbon double bond which is evidently of the unreactive type present in triterpenes. The results of the investigation so far carried out agree with the classification of castanogenin as a new member of the triterpenoid acid class of sapogenins.