Jasmonic acid and methyl jasmonate have been shown previously to be powerful inducers of proteinase inhibitors in tomato, tobacco, and alfalfa leaves. We show here that when proposed octadecanoid precursors of jasmonic acid, i.e., linolenic acid, 13(S)-hydroperoxylinolenic acid, and phytodienoic acid, were applied to the surfaces of tomato leaves, these compounds also served as powerful inducers of proteinase inhibitor I and II synthesis, a simulation of a wound response. By contrast, compounds closely related to the precursors but which are not intermediates in the jasmonic acid biosynthetic pathway did not induce proteinase inhibitor synthesis. These results suggest that the octadecanoid intermediates may participate in a lipid-based signaling system that activates proteinase inhibitor synthesis in response to insect and pathogen attack.