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Bacterial diseases cause tilapia's high-mortality outbreak. This study investigated the toxicity of azithromycin ( AZT), a macrolide antibiotic that has been considered a possible therapeutic drug for tilapia aquacultural use. The 48-h acute toxicity (50% lethal concentration, LC50; 48 h) of AZT was determined for O reochromis niloticus. Thereafter, fish were exposed to 0, 1, 50 and 100 mg L−1 AZT during 14 days (chronic exposure) and measured the haematological variables, the activity of superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferase ( GST) and the concentration of glutathione ( GSH), protein carbonyl and lipid peroxidation in the liver; histopathology was analysed the liver, gills and kidneys. The LC50; 48 h was >100 mg L−1. No fish died during chronic exposure. Haematocrit and haemoglobin concentration increased in fish exposed to 50 and 100 mg L−1, and the total number of leucocyte and thrombocyte increased after exposure to 100 mg L−1 AZT, suggesting a stimulation of defence cell production. In the liver, the antioxidant enzyme activities did not change, but GST activity and the GSH level increased in fish exposed to 100 mg L−1 AZT. Oxidative stress did not occur. Histopathological index ( HI L) indicates moderate liver damage; minor histological changes in the gill and no change in the kidneys. AZT was considered non-toxic for O. niloticus after acute exposure and, although it causes moderated histopathology in the liver after chronic exposure, this antibiotic may be an alternative against bacterial infections, depending on its efficacy to control bacterial disease in fish. [ABSTRACT FROM AUTHOR]