This paper describes the effect of a plant-derived polygalacturonase-inhibiting protein (PGIP) on the activity of endopolygalacturonases isolated from fungi. PGIP's effect on endopolygalacturonases is to enhance the production of oligogalacturonides that are active as elicitors of phytoalexin (antibiotic) accumulation and other defense reactions in plants. Only oligogalacturonides with a degree of polymerization higher than nine are able to elicit phytoalexin synthesis in soybean cotyledons. In the absence of PGIP, a 1-minute exposure of polygalacturonic acid to endopolygalacturonase resulted in the production of elicitor-active oligogalacturonides. However, the enzyme depolymerized essentially all of the polygalacturonic acid substrate to elicitor-inactive oligogalacturonides within 15 minutes. When the digestion of polygalacturonic acid was carried out with the same amount of enzyme but in the presence of excess PGIP, the rate of production of elicitor-active oligogalacturonides was dramatically altered. The amount of elicitor-active oligogalacturonide steadily increased for 24 hours. It was only after about 48 hours that the enzyme converted the polygalacturonic acid into short, elicitor-inactive oligomers. PGIP is a specific, reversible, saturable, high-affinity receptor for endopolygalacturonase. Formation of the PGIP-endopolygalacturonase complex results in increased concentrations of oligogalacturonides that activate plant defense responses. The interaction of the plant-derived PGIP with fungal endopolygalacturonases may be a mechanism by which plants convert endopolygalacturonase, a factor important for the virulence of pathogens, into a factor that elicits plant defense mechanisms.