QS-21, a natural saponin adjuvant, is obtained from the bark of the South American tree Quillaja saponaria Molina. It is an immunological adjuvant added to vaccines and other immunotherapies that is designed to enhance the body’s immune response to the antigen contained within the treatment. QS-21 has been proved to be safe for parenteral administration and represents a potential alternative to bacterial enterotoxin derivatives as a mucosal adjuvant. QS-21 appears to increase potency of the vaccine with relatively small quantities of antigen and to exhibit synergy with other adjuvants. The high level of purity and standardization of the QS-21 saponin adjuvant has enabled evaluation of this compound in clinical trials of experimental vaccines.