Abstract. The induction of proteinase inhibitor (PrI) activity in young tomato plants by wounding and oligosaccharides has been shown to be prevented by pretreatment of the plants with phenolic acids such as aspirin. Aspirin applied over a wide range of concentrations is able to inhibit PrI induction by pectic fragments. The possibility that other weak acids may also inhibit PrI induction was investigated. Isobutyric acid and trimethylacetic acid were shown to be less effective as inhibitors than aspirin, and weak bases were without effect. However, it was demonstrated that various agents known to influence ATPase activity and intracellular pH were able to inhibit PrI induction, and in particular the striking effect of low concentrations of fusicoccin on PrI induction was noted.