Juvenile rainbow trout Oncorhynchus mykiss (Walbaum) were exposed to therapeutic, and higher concentrations of chloramine-T (Cl-T) to assess the effects of this chemical on the antioxidant enzyme system and genetic structure. Red blood cells acetylcholinesterase, increment -aminolevulinic acid dehydratase, paraoxonase and liver glutathione S-transferase activity were increased at 10 and 20mg L-1 Cl-T-exposed fish, while they were decreased at 30mg L-1 Cl-T-exposed fish. On the other hand, liver catalase activity and liver protein levels increased at 10mg L-1 and decreased at 20 and 30mg L-1 concentrations of Cl-T. Liver super-oxide dismutase activity decreased at 10mg L-1 and 20mg L-1 Cl-T and increased at 30mg L-1 of Cl-T. Compared to control, comet assay indicated that Cl-T did not cause significant DNA damage to red blood cells of the fish. Results indicate that 10 or 20mg L-1 Cl-T can be safely used to prevent or treat external parasitic and bacterial infection of rainbow trout.