Fifteen patients with serious infections due to Pseudomonas aeruginosa were treated with azlocill in a new ureidopenicillin. Fourteen out of fifteen bacterial isolates were inhibited (mic) by 32 mcg/ml. or less of the drug. The mean blood concentration of the drug one hour after each i.v. dose was 159.6 mcg/ml. Fourteen (93%) out of the fifteen patients were Clinically and bacteriologically cured. The drug was well tolerated.