Serum bactericidal titres following a 200 mg i.v. dose of ciprofloxacin were measured in healthy volunteers and compared with those achieved with standard doses of ceftazidime, piperacillin and gentamicin, given alone or in combination. Five strains of each of four enterobacterial species were included in the study. Bactericidal titres 1 h after infusion of ciprofloxacin exceeded 1:16 for Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris and Enterobacter cloacae. While ceftazidime produced higher peak bactericidal titres against the Enterobacteriaceae, piperacillin was less effective than ciprofloxacin against three of the four enterobacterial species tested. Among the combinations, only piperacillin/gentamicin showed synergistic activity against some strains. Studies of bacterial killing kinetics again confirmed the high bactericidal activity of ciprofloxacin for the Enterobacteriaceae. The combination of ciprofloxacin with gentamicin resulted in more rapid killing of some strains of Klebsiella pneumoniae and Proteus vulgaris.