The bactericidal activity of cefazolin, cefoxitin, and cefmetazole against clinical isolates of Escherichia coli and Klebsiella pneumoniae was investigated. The mean geometric minimum inhibition concentrations against 200 strains each of the test organisms were lowest for cefmetazole, followed by cefazolin and cefoxitin. The killing activity at 5 and 50 μg/ml of cefazolin and cefmetazole was almost the same and was superior to that of cefoxitin. In the kinetic model under conditions simulating the serum levels of the two drugs in humans after intravenous injection (1 g), cefazolin was the strongest of the three drugs in bactericidal activity. The results indicate that the highest and most prolonged serum concentrations of cefazolin reflected the strong bactericidal activity and the longest inhibition period of bacterial regrowth