Abstract The response of tomato plants to injury or to treatment with cell wall fragments has been studied. Wounding, pectic fragments or chitosan causes the systemic accumulation of proteinase inhibitor proteins. This response can be inhibited by pretreatment of the plants by aspirin. The effect of aspirin is rapid and reversible. Related hydroxybenzoic acids are also inhibitory: the structural specificity required for inhibition is very similar to that demonstrated by others to be required for induction of pathogenesis-related proteins in tobacco.