Quillaja saponaria Molina is a South American tree native to Chile whose bark is rich in a complex mixture of triterpenic saponins. Quillaja-derived saponins, such as Quil-A®, are currently used as veterinary vaccine adjuvants. One specific fraction of the Quil-A saponin mixture, QS-21, is an extremely potent adjuvant with potential use in a broad variety of vaccines. It has been shown to enhance antibody- and cell-mediated immune response and has been investigated for human vaccine applications. QS-21 possesses an optimal balance of low toxicity and high immunostimulatory efficacy, and has already been used in one commercialised vaccine.