The Proteinase Inhibitor Inducing Factor, PIIF, a pectic polysaccharide that induces synthesis and accumulation of proteinase inhibitor proteins in tomato and potato leaves, is an effective elicitor of the phytoalexin pisatin in pea pod tissues. The levels of pisatin induced by PIIF, and the time course of elicitation, are similar to those induced by chitosans, beta-1,4 glucosamine polymers, which are potent elicitors of pisatin in pea pods. Similarly, the chitosans, found in both insect and fungal cell walls, are the most potent inducers yet found of proteinase inhibitor accumulation in excised tomato cotyledons. The similarity in the induction of synthesis of proteinase inhibitors in tomato cotyledons and of pisatin in pea pods by pectic polysaccharides and chitosans suggests that the two polysaccharide types may be triggering a similar fundamental system present in pea and tomato plants that regulates the expression of genes for natural protection systems.