The activities of azlocillin and mezlocillin were compared with those of carbenicillin, ticarcillin, and pirbenicillin against a wide range of gram-negative organisms. The two new drugs were considerably more active than carbenicillin against Klebsiella species and Escherichia coli. Carbenicillin was twice as active against Proteus mirabilis as mezlocillin and four times as active as azlocillin. Against Pseudomonas aeruginosa, azlocillin was eight times as active as carbenicillin. Azlocillin and mezlocillin were twice as active as carbenicillin against Bacteroides fragilis, and these drugs showed a high degree of activity against Haemophilus influenzae and Neisseria gonorrhoeae.