Saponins are natural surfactant compounds whose aqueous solutions can form stable foam and micellar solutions. They are glycosides formed by one, two or three oligosaccharide chains linked to a steroid or triterpene aglycone through acetal and/or ester-acetal bonds. Quillaja brasiliensis is an endemic tree species in southernBrazil and northern Uruguay. It has been shown in experimental vaccines in animal models that the aqueous extract and some of the purified saponin fractions obtained from leaves of Q. brasiliensis have immunoadjuvant activity comparable with Quil-A®, the main commercial adjuvant product based on Q. saponaria saponins [1, 2]. These saponins are also able to form ISCOM- type nanometric micellar structures which are even more effective vaccine adjuvants, generating both humoral andcellular immune responses [2]. The aim of this study was to perform a preliminary structural characterization of the immunoadjuvant fraction of saponins designated as Fraction B.