Sera of volunteers receiving 1 g mezlocillin, 5 g mezlocillin, 1 g ceftazidime, 3 g ceftazidime, 1 g mezlocillin plus 1 g ceftazidime, and 1 g mezlocillin plus 500 mg amikacin, respectively, were evaluated for bactericidal activity against clinical isolates of Klebsiella pneumoniae and Pseudomonas aeruginosa. The titers of bactericidal activity against Klebsiella pneumoniae in serum from subjects receiving ceftazidime were higher than with other regimens both one and six hours after administration. Peak titers of bactericidal activity greater than or equal to 1:8 were also achieved more often against Pseudomonas aeruginosa in sera from subjects receiving ceftazidime than with other regimens. Killing studies confirmed these results. Although the checkerboard technique indicated synergism with the combination mezlocillin plus amikacin in vitro, this was not confirmed in vivo. Single drug therapy with ceftazidime was superior to the tested combinations.