This work studies the inhibitory effects and fate of the antibiotics oxytetracycline (OTC) and chlortetracycline (CTC) during the anaerobic digestion of pig manure. Both substances were added together in batch assays at concentrations of 10, 50 and 100 mg L(-1). Control assays only with antibiotics (abiotic) as well as without antibiotics (biotic) were also conducted. Methane production was reduced by 56%, 60% and 62% at OTC and CTC concentrations of 10, 50 and 100 mg L(-1), respectively. The IC(50) level calculated from these experiments was estimated to be around 9 mg L. a significant value considering the reported concentrations of these compounds in pig manure samples (up to 136 mg L(-1)). Strong adsorption to solid matter was observed, which increased the stability of both substances. Antibiotic degradation was thus much higher in control assays, without solids, than those determined from assays including inoculum and manure substrate. (C) 2010 Elsevier Ltd. All rights reserved.