Multidrug resistance is expressed not only by bacteria, but also by tumor cells and by some normal cells of the body. It enables eukaryotic cells to exclude not only cytostatic drugs but also non-cytostatic antibiotics. This was demonstrated in genetically engineered multidrug resistant (MDR) cells infected with the facultative intracellular bacterium Listeria monocytogenes for all macrolide antibiotics tested (azithromycin, clarithromycin, erythromycin, josamycin, roxithromycin and spiramycin). In these cells and in conventionally selected MDR cells higher concentrations of the macrolides were necessary to inhibit the growth of L. monocytogenes than in the respective parental cells. This effect was due to a reduced intracellular accumulation, which was shown with a biological assay for all macrolides tested. For azithromycin, the results of this test were confirmed by measurement of the intracellular concentrations with high-performance liquid chromatography (HPLC). Besides the macrolides, MDR cells excluded also antibiotics of other chemical groups which was shown for ciprofloxacin, clindamycin, rifampicin and the streptogramin derivative RP 59500. In addition, in conventionally selected cells higher concentrations of chloramphenicol, doxycyclin, ofloxacin and trimethoprim than in the respective parental cells were necessary to inhibit the growth of L. monocytogenes. In contrast, when using genetically engineered cells, no significant differences were found for these antibiotics. These differences might be due to a higher expression of multidrug resistance in the conventionally selected cells because these cells were also more effective in excluding rhodamine 123 in a flow cytometric assay. In conclusion, expression of multidrug resistance by eukaryotic cells leads to a reduced concentration of macrolides and other antibiotics in these cells and to an impairment of activity against intracellular bacteria.