Peptide isomerase catalyses the post-translational isomerisation of the l- to the d-form of an amino acid residue around the N/C-termini of substrate peptides. To date, some peptide isomerases have been found in a limited number of animal secretions and cells. We show here that papaya extracts have weak peptide isomerase activity. The activity was detected in each 30-100 kDa fraction of the flesh and the seed extracts of unripe and ripe papaya fruit. The definitive activity was confirmed in the ripe papaya extracts, but even then it was much less active than that of the other peptide isomerases previously reported. The activity was markedly inhibited by methanol, and partly so by amastatin and diethyl pyrocarbonate. This is the first report of peptide isomerase activity in a plant and suggests that perhaps every living organism may have some peptide isomerase activity.