excellent activity of newer quinolones on legionella pneumophila in j774 macrophages the activity of six antibiotics directed against intracellularly multiplying legionella pneumophilia was examined in tissue cultures with j774 macrophages the drugs tested were the new quinolones bay y 3118 and clinafloxacin and ciprofloxacin erythromycin gentamicin and ampicillin served as reference drugs additionally the mics of these drugs against l pneumophila were determined in vitro by broth microdilution despite their low mic values ampicillin and gentamicin did not inhibit intracellular multiplication of l pneumophila in j774 macrophages in contrast an inhibition of intracellular growth could be demonstrated for the four other antibiotics the new quinolones bay y 3118 and clinafloxacin showed the highest activity against intracellular l pneumophila at a concentration of 000078 mgl already a marked reduction in bacterial counts was seen for both drugs in comparison to the growth control without antibiotics the corresponding effective concentrations were 00125 mgl for ciprofloxacin and 02 mgl for erythromycin it may be concluded that new quinolone derivatives might become an alternative to erythromycin and rifampicin which at present are the drugs of primary choice for the treatment of legionnaires disease