The detection of antibiotics and resistant bacteria in sewage treatment plants (STPs) has stimulated a discussion on the origin and selection of resistant bacteria during sewage treatment. Currently, there is little data available regarding the effects of realistic mixtures of antibiotics on the bacteria present in the aeration tanks of STPs. In this study we used four laboratory-scale sewage treatment plants (LSSTPs) to study the effects of antibiotics on bacteria during sewage treatment under standardized conditions. Two plants were fed with a mixture of antibiotics at two concentration levels based on the average annual input of antibiotics into German municipal STPs. The total operational period was 84 days. A multiresistant bacterium (Acinetobacter baumannii) was added twice to two of the plants. The fate of the multiresistant bacterium was monitored. The mix of antibiotics did not affect the purification efficiency. The presence of the antibiotics did not favour the multiresistant bacterium. No difference was detected between the test plant and the controls.