Resistance of soybean (Glycine max L.) seedlings to Phytophthora megasperma var. sojae (Pms) is in part due to the accumulation in infected tissue of a compound which is toxic to Pms. The accumulation of this compound, a phytoalexin called glyceollin, is triggered by infection, but it can also be triggered by molecules, "elicitors," present in cultures of Pms. The ability of the Pms elicitor to stimulate phytoalexin accumulation in soybean tissues has been used as the basis for biological assays of elicitor activity. Two bioassays were developed and characterized in this study of the Pms elicitor. These bioassays use the cotyledons and the hypocotyls of soybean seedlings. The cotyledon assay was used to characterize the extracellular Pms elicitor. This elicitor was isolated from Pms cultures and purified by ion exchange and molecular sieving chromatography. The extracellular Pms elicitor was determined to be a predominantly 3-linked glucan, which is similar in composition and structure to a polysaccharide component of Pms mycelial walls.