Natural polysaccharides were examined for their activity as elicitors of phytoalexins and flavonoids in alfalfa cotyledons. Only two polysaccharides, laminaran and pectic acid, had elicitor activity. Laminaran, which was more active than pectic acid, was hydrolyzed by tunicase and the hydrolysate was subjected to charcoal and gel filtration columns. Introduction of the pyridylamino group into the elicitor-active oligosaccharides was attempted in order to facilitate isolation. The pyridylamino derivatives were found to exhibit higher activity than the original oligosaccharides. Their liquid chromatography mass spectrometry (LC-MS) analysis revealed that the elicitor-active principles form two ion clusters with the same molecular weights, m/z 1070 and 1232. Their high performance liquid chromatography (HPLC) analysis showed three main peaks. The individual peaks (LN-1, 2,3) were collected and subjected to the alfalfa cotyledon assay. LN-3 showed the highest activity (minimum effective concentration, 0.8 mug/ml).