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. It was originally isolated and named by Dr Charlotte Kensil in 1991 and among the components of triterpenoid quillaja saponins, QS-21 possesses an optimal balance of low toxicity and high immunostimulatory efficacy, and has already been used in one commercialised vaccine.