The immunological response to the killed organism or product thereof has been enhanced by the use of adjuvants. Although the most common adjuvant for animal studies has been the complete Freund’s adjuvant, it cannot be used in humans because it causes liver, skin and spleen dysfunction. The most common adjuvant for humans is aluminium compounds, which are generally safe for human use. Others include muramyl dipeptide, biodegradable polymers, and a glycoside adjuvant called Quil A from the bark of an Amazon oak tree. However, many others are being developed or will probably be given U.S Food and Drug Administration (FDA) acceptance in the future. The key feature will be their immunogenic enhancement and their strength of safety for use in humans.