The component in Viscum album extract Iscador-M enhancing the NK cytotoxicity of human CD56+CD3- NK cells (87-95% enrichment) in cocultures with K562 tumor cells and increasing the formation of NK cell/tumor cell conjugates was identified as a rhamnogalacturonan. Both activities were abolished by treatment of V. album extract with poly-alpha-D-galacturonidase and alpha-rhamnosidase and both activities were inhibited in the presence of galacturonic acid and acetylated rhamnose (6-deoxymannose). Inhibition was also observed in the presence of structurally related derivatives such as acetylated mannose or acetylated mannonic acid gamma-lactone, the latter exhibiting a 5-10-fold higher inhibitory potential. The rapid formation of NK cell/tumor cell conjugates in the presence of V. album extract was based on the bridging of NK cells with tumor cells by rhamnogalacturonan. Using a specifically adapted agglutination assay, the saccharide residues of the rhamnogalacturonan interacting with NK cells and tumor cells could be identified by the formation of homologous cell conjugates induced by acetylated rhamnose or acetylated mannose conjugated to dextran and by polygalacturonic acid: terminal acetylated rhamnose or acetylated mannose bound only to NK cells in a dose-dependent manner but not to K562 tumor cells, and terminal galacturonic acid only to K562 tumor cells but not to NK cells. This type of bridging represents a novel mechanism of enhancement of NK cytotoxicity.