The interaction between the fungal leaf pathogen Cladosporium fulvum and its only host, tomato, fits the gene-for-gene model. In tomato, the Cf-9 resistance gene product mediates specific recognition of the fungal avirulence gene product AVR9, resulting in a hypersensitive response and resistance. Cf9 tomato leaves respond with necrosis after injection with AVR9, whereas Cf9 tomato cell suspensions do not show defence responses after treatment with AVR9. Here we report on early defence responses induced in Cf-9 transgenic tobacco leaves and Cf-9 transgenic tobacco cell suspensions after treatment with synthesized AVR9 and mutant analogues R08K, F10A and F21A. The necrosis-inducing activity of the AVR9 peptides increased in the order F21A, F10A, AVR9, R08K. An oxidative burst was induced at a much lower AVR9 peptide concentration as compared to medium alkalization and necrosis. Interestingly, the mutant peptide F21A failed to induce necrosis and medium alkalization but did induce an oxidative burst. In all assays, the relative differential activities of the AVR9 peptides were similar to those observed in Cf9 tomato leaves. Both AVR9 and F21A activated a MAP kinase in Cf-9 transgenic tobacco cell suspensions. AVR9 also induced specific cell death in these suspensions. The relation between the induction of early defence responses and necrosis is discussed.