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A method consisting of solid phase extraction followed by ultra-high performance liquid chromatography combined with time-of-flight mass spectrometry (UHPLC-ToFMS) has been developed for the determination of multiple-class antibiotics in influents and effluents of wastewater-treatment-plants (WWTPs). The nineteen analyzed antibiotics belong to the 5 most prescribed antibiotic classes in the Czech Republic, namely tetracyclines, macrolides, sulfonamides, fluoroquinolones, and lincosamides. The matrix-matched calibration technique with internal standard addition for each antibiotic group was employed for quantification. Two extractions employing an Oasis HLB cartridge and two injections were needed for each sample due to the different physico-chemical properties of the tested antibiotics. Prior to the extraction on the Oasis HLB SPE cartridge, the samples were adjusted to pH 4.5 and 7.5. The usefulness and versatility of the method was documented by achieving method limits of quantification up to 10 ng L-1 and recoveries >80% for most analytes. The method was used for analysis of water samples of WWTPs from 6 localities in the Czech Republic in order to reveal the occurrence of selected antimicrobial agents and assess the efficiency of WWTPs in the removal of these antibiotics. All tested samples were positive for antibiotics with concentrations ranging from 5 ng L-1 to 1290 ng L-1. Antibiotics were present in both influent and effluent water samples, which documented the poor removal efficiency of the WWTPs.