Mezlocillin is a new acyl-ureido-penicillin with a broad spectrum of action, particularly directed against Gram-negative bacteria, including Pseudomonas aeruginosa. The therapeutic efficacy of this new antibiotic was assayed in 20 patients affected by serious biliary tract infections. Complete recovery was reached in 80% of the patients. Such a recovery rate is very high, considering the type of infectious pathology. Furthermore, the pharmacokinetics of mezlocillin was followed in the bile, as well as in the wall of the gallbladder, in gall-stones and in serum after single or repeated administration of the drug; the results favored the therapeutic use of mezlocillin. It is concluded that mezlocillin is a first choice antibiotic in the treatment of biliary tract infections.