Suspension-cultured cells of Lycopersicon peruvianum L. reacted to the presence of mechanically damaged cells with a transient alkalinization of their culture medium. This response resembled the alkalinization observed after treatment with fungal signal molecules such as chitin fragments and ergosterol or after application of the protein phosphatase inhibitor calyculin A. When compounds implicated in wound signalling were tested, the 18 amino acid peptide systemin was found to be a potent inducer of the alkalinization response, with a half-maximal activity at concentrations of ∼100 pM. The decrease in extracellular H+ was paralleled by an increase of K+, and induction of both ion fluxes was blocked by the protein kinase inhibitor K-252a. Systemin also caused rapid increases in the activities of 1-aminocyclopropane-1-carboxylate (ACC) synthase and phenylalanine ammonia-lyase, two other responses commonly observed in cells treated with elicitors. The systemin analogue systemin-Ala17, a reported systemin antagonist in the induction of proteinase inhibitors in tomato plants, provoked a much weaker alkalinization response and did not induce ACC synthase at all. When applied together with authentic systemin, this analogue antagonized induction of both responses, indicating that the perception system for systemin had very similar properties in the L. peruvianum cells as in tomato plants. In conclusion, suspension-cultured L. peruvianum cells provide a convenient and highly sensitive system to study elements of wound response and, in particular, systemin perception.