Environmental analytical studies show that trace concentrations of antibacterial agents (antibiotics) occur in hospital and municipal wastewaters and in the aquatic environment. Fluoroquinolones and macrolides, two important human-use antibiotic classes, were studied in detail. The results are discussed regarding input sources and behavior in wastewater treatment and rivers. The fluoroquinolones ciprofloxacin and norfloxacin are substantially eliminated in wastewater treatment (80–90%) by sorption transfer to sewage sludge. In digested sludges the fluoroquinolones occur at mg/kg levels. Ciprofloxacin and norfloxacin are further removed in the Glatt river by 66 and 48%, respectively. The most abundant macrolide clarithromycin was detected at 57 to 330 ng/l concentrations in treated wastewater effluents. Different compositions of the macrolides (clarithromycin and erythromycin-H2O) determined in treated effluents of three wastewater treatment plants can be explained by distinct consumption patterns, in one case due to an international airport located in the catchment area. Residual levels of clarithromycin in the Glatt river were up to 75 ng/l with no apparent removal in the river. These results provide important information on environmental exposures, which can be incorporated into environmental risk assessments of the particular chemicals.