To study the anti-infectious effect of a vascular allograft, the antimicrobial activity of tryptophan metabolites mediated by indoleamine 2,3-dioxygenase was determined. The growth of methicillin-resistant Staphylococcus aureus (MRSA) over 10 h in extracts from post-transplantation vascular allograft was significantly slower than that of extracts from non-transplantation vascular allograft regardless of the presence of tryptophan (p<0.05). When the antimicrobial activity of the tryptophan metabolites in the L-tryptophan-L-kynurenine pathway was examined, 3-hydroxy-DL-kynurenine and alpha-picolinic acid had strong antibacterial activity against MRSA, S. epidermidis, Escherichia coli, and multidrug-resistant Pseudomonas aeruginosa, although antimicrobial activities of anthranilic acid, 3-hydroxyanthranilic acid, and quinolinic acid against them were low. The results showed that, of the tested tryptophan metabolites, 3-hydroxy-DL-kynurenine and alpha-picolinic acid contributed to the anti-infectious effects of the allograft by inhibiting of the growth of microorganisms.