The aim of this study was to determine the fate of veterinary antibiotics entering biological treatment process. Due to the prevalence of their respective antibiotic family usage in livestock, tetracycline and tylosin were selected. Using modified Sturm test (OECD 301-B), their biodegradation were compared to that of a referent pollutant, sodium benzoate, well-known for its high biodegradability. Biodegradation rates were -28 and -35% for tetracycline and 4 and -5% for tylosin showing an absence of biodegradability. OECD 301-B inhibition tests showed a potential toxicity of both molecules on activated sludge inoculum derived from membrane bioreactor. Tetracycline presented good adsorbability while tylosin remained mostly present in the soluble phase. The Langmuir maximum adsorption capacity (C(s,max)) was found to be 72 and 7.7 mg g(-1) for tetracycline and tylosin, respectively. Adsorption was therefore the most favourable fate for tetracycline entering a biological process. Conclusions on tylosin case were more controversial. (C) 2009 Elsevier Ltd. All rights reserved.