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Tetracycline was exposed to ozone by bubbling at different pH levels, and the acute toxicity of ozonation byproducts was tested by luminescent bacteria (Vibrio qinghaiensis sp.-Q67). Residual tetracycline in the solutions was detected by high performance liquid chromatography (HPLC). The results indicated that tetracycline was immediately degraded by ozonation and the degradation was favored at higher pH. Tetracycline (20  mg L−1) was completely degraded after 5  min of ozonation. However, total organic carbon (TOC) hardly decreased even though the ozonation time was extended to 40  min, suggesting that the ozonation process could not mineralize tetracycline. The luminescent bacteria assay indicated that the toxicity of the tetracycline solution was changed after ozonation. Chemical oxygen demand (COD) dropped by 15%, and the BOD5/COD ratio increased from 0·09 to 0·26, suggesting that the biodegradability of tetracycline was enhanced after 20 min of ozonation treatment.