The human immune system, despite having its own sophisticated defence mecha­ nisms, is inferior to bacteria and viruses with respect to adaptability. Furthermore, our immune system is increasingly exposed to detrimental effects, that is immuno­ suppressive environmental consequences, unhealthy living, and chronic illnesses. Excessive chemotherapy threatens our immune system even further. This situation demands compensatory prophylactic therapeutic regimes. One of these - specific immunostimulation - is more difficult to achieve than the immunosuppression cur­ rently used in transplantation surgery and the medical treatment of autoimmune dis­ eases. The earliest attempts to develop suitable medication for immunostimulation were based on traditional remedies which embodied the accumulated experience of several centuries. Medicinal plants are already being used prophylactically as stan­ dardized and efficacy-optimized preparations for the treatment of various recur­ rent infections, or in combination with chemotherapeutics in standard medical practice. In order to rationally apply immunostimulants of plant origin, however, it is necessary to search for the active principles of these substances and to produce them in a pure form. Because suitable screening methods have become available only recently, research in this field is in its very beginning. Further progress can be expected from systematic basic research on the mechanisms underlying immunomodulation. This also applies to verification of clinical efficacy, which is a prerequisite for the acceptance of medications with purported immunostimulatory properties.