Membrane fouling remains the major obstacle for development of membrane bioreactors (MBRs). This paper aimed at investigating the effects of carbamazepine (CBZ) present in a discontinuous way at high concentration (peak injection) on fouling propensity of activated sludge from a MBR that treats domestic wastewater. Batch experiments with peak injection of CBZ (100 mu g L-1 in sludge for 3 h) were performed for sludges sampled from a MBR operated under different organic loading conditions. HPLC-SEC analysis with a fluorescence detector was employed to study the effects of CBZ on protein-like compounds in supernatant. With addition of CBZ in sludge, a significant increase of sludge fouling propensity was observed when using sludge sampled from the MBR operated under low organic loading rate (0.1 kg COD/ kg EVESS/d), which could be attributed to the increase in the quantity of protein-like substances with a 100-1000 kDa molecular size in supernatant. Whereas reduced CBZ effect was found for sludge sampled from the MBR operated under higher organic loading rate (0.2 kg COD/kg MLSS/d). Whatever the sludge, the injection of CBZ induced no significant change of the floc size distribution and of the polysaccharide concentration in supernatant. Moreover, it did not affect the sludge microbial activity.