The notion that medicines derived from plants depend for their action on active principles present has to be modified in view of the findings that there are, in many cases, adjuvant substances in the plant which enhance the activity of the components actually responsible for the effect. This synergy may involve protection of an active substance from degradation by enzymes; it may facilitate transport across barriers such as cell and organelle walls, it may overcome multi-drug resistance mechanisms or provide other signals to the host's cells that result in higher efficacy of the crude drug when compared with isolated components. The many plant substances that stimulate the immune system, often at very low doses, have not been reviewed as this is not strictly speaking synergy. Some of the evidence for the phenomena described is reviewed and its bearing on phytotherapy commented.