Discharge of chemical compounds in pharmaceuticals and personal care products (PPCPs) without appropriate treatment is an environmental problem. Ammonia-oxidizing bacteria (AOB) oxidize ammonia to nitrite and have also been reported to degrade recalcitrant organic compounds by co-metabolism. In this study, Nitrosomonas europaea, a typical ammonia-oxidizing bacterium, was tested against the antibiotic clarithromycin (CAM) and the anticonvulsant carbamazepine (CBZ) and was found to degrade CAM but not CBZ. The pseudo-first-order degradation rate constant of N. europaea for CAM was at maximum one order of magnitude higher than that of activated sludge, showing the feasibility of biodegredation of CAM by N. europaea. Respirometric assay indicated that N. europaea was inhibited at a CAM concentration above 10 mu g/L. The microarray for gene expression demonstrated the presence of upregulated genes for ammonia oxidation and ribosome synthesis.