The presence of antibiotics in water resources has been disturbing news for the stakeholders who are responsible for public health and the drinking water supply. In many cases, biological wastewater treatment plants are the final opportunity in the water cycle to trap these substances. The sensitivity of activated sludge bacteria to erythromycin, a macrolide widely used in human medicine was investigated in batch toxicity tests using a concentration range of 1-300 mg L-1. Erythromycin, a protein synthesis inhibitor, has been found to significantly inhibit ammonification, nitritation and nitratation at concentrations higher than 20 mg L-1. The degree of inhibition increased with greater concentrations of the antibiotic. Exposure to erythromycin also clearly affected heterotrophs, particularly filamentous bacteria, causing floc disintegration and breakage of filaments. Cell lysis was observed with the concomitant release of organic nitrogen (intracellular proteins) and soluble COD. Although erythromycin exhibits properties of a surfactant, this characteristic alone cannot explain the damage to heterotrophs: the effects from erythromycin were greater than those of Tween 80, a commonly used surfactant. Floc disruption can lead to the release of isolated bacteria, and possibly antibiotic resistance genes, into the environment. (C) 2009 Elsevier B.V. All rights reserved.