In a previous report [Mol. Gen. Genet. 228 (1991) 281], carboxypeptidase inhibitor protein (CPI) mRNA was found to accumulate in leaves of wounded tomato plants, but CPI protein could not be detected. In contrast, we found that CPI protein does accumulate in tomato leaves in response to wounding, and also in response to treatment with either systemin, methyl jasmonate (MeJ), oligogalacturonic acid, or chitosan. Identification of CPI protein was confirmed by its inhibition of metallo-carboxypeptidase A (CPAase), which was used as an assay during purification of the inhibitor from leaves of MeJ-treated tomato plants. Amino acid sequence analysis and mass spectroscopic analyses of the pure protein confirmed its identity as CPI. The pure protein inhibited CPAase in a 1:1 stoichimetric interaction. Time course analyses of the induction of CPI mRNA in tomato leaves in response to wounding indicated that the gene is a member of the group of "late genes" that code for defensive proteins synthesized in leaves in response to herbivore attack.