Bayer developed MEZLOCILLIN (previously known as BAYPEN); it is a semisynthetic ampicillin-derived penicillin. Mezlocillin is a penicillin beta-lactam antibiotic used in the treatment of bacterial infections caused by susceptible, usually gram-positive, organisms. The bactericidal activity of mezlocillin results from the inhibition of cell wall synthesis and is mediated through mezlocillin binding to penicillin binding proteins (PBPs). Mezlocillin is stable against hydrolysis by a variety of beta-lactamases, including penicillinases and cephalosporinases and extended spectrum beta-lactamases. Mezlocillin was poorly absorbed orally and was given either intramuscularly or intravenously. This drug was discontinued in the U.S.