The MIC of the four quinolones were determined for 839 bacterial isolates. The mean in vitro activity of pefloxacin against strains susceptible to nalidixic acid was found to be four times greater than flumequin, eight times greater than pipemidic acid, sixteen times greater than nalidixic acid. On resistant strains pefloxacin, at levels lower than 32 micrograms/ml, remained active against 67 go 100 per cent of the strains as a function of bacterial species. Streptocoques D and Pseudomonas aeruginosa, resistant to other quinolones, were found to be susceptible to pefloxacin.