Several clinical studies have reported that an increase in excretion of tryptophan metabolites 3-hydroxyanthranilic acid (3-OHAA), anthranilic acid (AA) and other metabolites in the urine of bladder cancer patients are implicated to play a role in the etiology of bladder cancer; however the mechanisms involved are unknown. The present study compares the genotoxicity of tryptophan metabolites AA and 3-OHAA to cause mutagenesis in vitro. The DNA damage effects of tryptophan metabolites were analyzed using plasmid relaxation assay performed with AA and 3-OHAA at various concentrations between 50μM and 400μM in the presence of plasmid DNA pSP-72. Both AA and 3-OHAA did not show any plasmid relaxation activity when tested alone. However, 3-OHAA in the presence of metal cofactor Cu (II) induced plasmid relaxation by causing nicks in the plasmid. This effect was not observed in the presence of other metal cofactors Fe (II) and Mn (III). Cu (II) at increasing concentrations between 5μM and 20μM and in the presence of 100μM 3-OHAA showed an apparent dose-response in causing DNA strand breaks. The Cu (II) mediated mutagenic activation of 3-OHAA was further investigated using Ames Salmonella/microsome mutagenicity assay with reactive oxygen species (ROS) sensitive tester strain Salmonella TA102. When 100μg of 3-OHAA per plate was incubated with Cu (II) a significant increase in TA102 revertants was observed with an increase in the concentration of Cu (II) from 2.5μg to 50μg. In contrast, AA with Cu (II) at such low concentration was unable to cause any significant increase in number of the TA102 revertants. This evidence for mutagenicity with only 3-OHAA and Cu (II) but not AA suggests the presence of hydroxyl group at ortho position to amino group in 3-OHAA structurally, is critical in reacting with Cu (II) to generate genotoxicity.