Mezlocillin is a broad-spectrum penicillin antibiotic. It is active against both Gram-negative and some Gram-positive bacteria. Unlike most other extended spectrum penicillins, it is excreted by the liver, therefore it is useful for biliary tract infections, such as ascending colangitis. Like all other beta-lactam antibiotics, mezlocillin inhibits the third and last stage of bacterial cell wall synthesis by binding to penicillin binding proteins. This ultimately leads to cell lysis. Mezlocillin is a new broad-spectrum semisynthetic penicillin that has shown encouraging in vitro activity against the infecting organisms most likely to cause mortality and morbidity in cancer patients receiving chemotherapy. The serum clearances and urine recoveries of mezlocillin, ampicillin, and carbenicillin were compared after the intravenous administration of single 3-g doses.