Bacteriostatic and bactericidal effects of three fluoroquinolones, norfloxacin, ofloxacin and pefloxacin, against 320 strains of Gram-negative bacilli were studied in vitro. For nalidixic acid-susceptible Enterobacteriaceae, susceptibility or resistance to second or third generation cephalosporins has no significant bearing on the MIC 90 of each of the three antibiotics. This is not so of nalidixic acid-resistant strains. All E. coli and K. pneumoniae strains are susceptible to the three quinolones (MIC 90 less than 3 mg/1); MICs 90 are higher for E. cloacae and S. marcescens; Serratia strains in particular have an MIC 90 greater than 6 mg/1. Nalidixic acid-susceptible Acinetobacter strains can be eliminated by ofloxacin or pefloxacin, whereas norfloxacin has the greatest activity against P. aeruginosa. We conclude that when multiresistant bacteria emerge, testing of susceptibility to the three quinolones studied may be useful.