The individual and combined effect of cefazolin (CFZ) and oxytetracycline (OTC) on thermophilic anaerobic digestion (55 degrees C) of dairy manure in batch digesters was investigated. Methane productions from the concentrations tested (30, 60 and 90 mg L-1) were compared with no-antibiotic control. CFZ concentrations showed no inhibition (P > 0.05) for methane production. The individual OTC and combined CFZ and OTC at concentrations of 30, 60 and 90 mg L-1 represented 79.1%, 70.3%, 68.6% (P < 0.05) and 88.5%, 82.7%, 70.3% (P < 0.05) respectively, of the control values. The high CH4 production, optimal pH and VFA data during digestion indicated the process stability and treatment efficiency. (C) 2013 Elsevier Ltd. All rights reserved.