Much attention has been devoted recently to the fate of pharmaceutically active compounds such as tetracycline antibiotics in soil and water. Tetracycline (TC) biodegradability by activated sludge derived from membrane bioreactor (MBR) treating swine wastewater via CO(2)-evolution was evaluated by means of modified Sturm test, which was also used to evaluate its toxicity on carbon degradation. The impact of tetracycline on a semi-industrial MBR process was also examined and confronted to lab-scale experiments. After tetracycline injection in the pilot, no disturbance was detected on the elimination of organic matters and ammonium (nitrification), reaching after injection 88% and 99% respectively; only denitrification was slightly affected. Confirming the ruggedness and the superiority of membrane bioreactors over conventional bioreactors, no toxicity was observed at the considered level of TC in the pilot (20 mg TOC L(-1)), while at lab-scale sodium benzoate biodegradation was completely inhibited from 10 mg TOC L(-1) TC. The origin of the activated sludge showed a significant impact on the performances, since the ultimate biodegradation was in the range -50% to -53% for TC concentrations in the range 10-20 mg TOCL(-1) with conventional bioreactor sludge and increased to 18% for 40 mg TOCL(-1) of TC with activated sludge derived from the MBR pilot. This confirmed the higher resistance of activated sludge arising from membrane bioreactor. (C) 2008 Elsevier Ltd. All rights reserved.