The anti-pseudomonas activities of azlocillin and mezlocillin were compared with that of ticarcillin. We measured the minimal inhibitory and minimal bactericidal concentrations of the three drugs against 20 different strains of Pseudomonas aeruginosa and found significantly lower values for azlocillin than for the other two drugs. We then infused 5 g of each drug into 10 volunteers on three consecutive days and determined the serum levels of the three antibiotics at 1-h intervals from 1 to 6 h after injection. The levels of azlocillin were significantly higher than those of mezlocillin and ticarcillin (at 1 h: 236.55 mug/ml +/- 12.9 for azlocillin, 192.45 mug/ml +/- 28.8 for mezlocillin, and 131.5 mug/ml +/- 10.9 for ticarcillin). The inhibitory and bactericidal activities of the sera obtained 1 and 6 h after the injection against the same 20 strains of P. aeruginosa demonstrated a significantly greater anti-pseudomonas activity of azlocillin when compared with mezlocillin and ticarcillin; mezlocillin and ticarcillin had approximately the same activity. The mean values for bactericidal activity against the strains tested were 1/32 for azlocillin, 1/8 for mezlocillin, and 1/8 for ticarcillin. Azlocillin thus appears to be a promising anti-pseudomonas drug and should be tested in clinical trials.