BACKGROUND: CO2- evolution tests (OECD 301-B), formerly known as modified Sturm tests, are commonly used for evaluation of the biodegradation potential of non-volatile molecules through measurement of the carbon dioxide produced. Extension of the modified Sturm test to evaluation of the toxicity of veterinary antibiotics, tetracycline and tylosine, on the biological treatment of swine wastewater was examined in this work. RESULTS: Modified Sturm test results highlighted the care which should be given to the description of the operational parameters (matrix and inoculum source). Using lab-scale inoculum, it was shown that the toxicity threshold of tetracycline (7-10 mg L-1 total organic carbon (TOC)) was higher than that of tylosine (4-5 mg L-1 TOC). Tylosine was found to be slightly biodegradable while total non-biodegradability was found for tetracycline. CONCLUSION: The higher tetracycline biodegradability may be related to the molecular conformation of tetracycline and tylosine antibiotics, the former is constituted of four benzoic cycles hardly breakable, while tylosine is constituted of a central structure with attached glycosidic-like fragments which might easily be biologically degraded. The antibiotic inhibitory effect was lower using membrane bioreactor sludge instead of lab-scale pilot sludge, in agreement with the lower resistance compared with pilot scale or industrial scale sludge. (C) 2009 Society of Chemical Industry