The effect of antimicrobials in wastewater on the effectiveness of small-scale wastewater treatment is of concern, although the usual concentrations of pharmaceuticals and personal care products (PPCPs) are lower than those that might induce inhibitory effects in large-scale wastewater treatment. The effect of antimicrobials on the operation of a newly designed two-stage membrane bioreactor, which is advantageous for the removal of micropollutants, was investigated. Levofloxacin (5 mg L^-1) and/or clarithromycin (2 mg L^-1) in the feed wastewater decreased the nitrification rate by at least 40％ and promoted membrane fouling. The accumulation of antimicrobials in the sludge phase in the reactors would be the reason for the retardant adverse effects even after stopping the addition of the antimicrobials.