The detection of antibiotics and resistant bacteriain sewage treatment plants (STPs) has stimulated adiscussion on the origin and selection of resistant bacteriaduring sewage treatment. Currently, there is little dataavailable regarding the effects of realistic mixtures ofantibiotics on the bacteria present in the aeration tanks ofSTPs. In this study we used four laboratory-scale sewagetreatment plants (LSSTPs) to study the effects of antibioticson bacteria during sewage treatment under standardizedconditions. Two plants were fed with a mixture of antibioticsat two concentration levels based on the averageannual input of antibiotics into German municipal STPs.The total operational period was 84 days. A multiresistantbacterium (Acinetobacter baumannii) was added twice totwo of the plants. The fate of the multiresistant bacteriumwas monitored. The mix of antibiotics did not affect thepurification efficiency. The presence of the antibiotics didnot favour the multiresistant bacterium. No difference wasdetected between the test plant and the controls