Our recent screening program with higher plants has revealed many low m. w . compounds (e. g. alcaloids , quinones , terpenoids , phenolcarboxylic acids) and high m. w. compounds (e. g. polysaccharides, glycoproteins) with pronounced immunostimulating potential. The most promising low m.w. compounds are some cytostatic compounds, which are known for their direct antitumoral activities at high doses and exert immunostimulating effects when applied in minute doses. In the class of high m.w. compounds, primarily some complex acidic arabinogalact- ans or rhamnogalacturonans (e. g. Echinacea purp. , Achyrocline sat. , Urtica dioica) show significant immunostimulating activities in vitro and in vivo.