The major storage protein of jackbean (Canavalia ensiformis) has been purified by a protocol involving ammonium-sulphate precipitation, gel filtration and ion-exchange chromatography. The protein was shown by partial amino-acid-sequence data to be homologous to vicilin, a major storage protein of pea (Pisum sativum), and is thus a member of the family of legume 7S proteins exemplified by pea vicilin. This protein is thus referred to as jack-bean vicilin rather than "canavalin" or "precanavalin" as previously used. Other properties of the jack-bean vicilin (e.g. subunit relative molecular mass (Mr) and structure, resistance to proteolysis) show similarity to phaseolin, the major 7S storage protein ofPhaseolus vulgaris. Jack-bean vicilin contained no detectable α-mannosidase activity, either as isolated from mature or germinating seeds, or after proteolytic treatment. α-Mannosidase was also purified from jack beans, and was shown to have a subunit Mr of approx. 120,000; it was separated completely from jack-bean vicilin by a similar protocol to that used for purifying the latter. The α-mannosidase was proteolytically cleaved after seed germination, but did not give polypeptides of the same Mr as jackbean vicilin. It was concluded that α-mannosidase and jack-bean vicilin are not related proteins.