(CLX), Tetracycline (TC), Erythromycin (ERY) and Sulfathiazole (ST) on methane-producing archaea (MPA) and sulfate-reducing bacteria (SRB) in anaerobic sludge was investigated using acetate or ethanol as substrate. With antimicrobial concentrations below 400 mg L1, the relative specific methanogenic activity (SMA) was above 50%, so that the antimicrobials exerted slight effects on archaea