Hetacillin is a penicillin beta-lactam antibiotic used in the treatment of bacterial infections caused by susceptible, usually gram-positive, organisms. The name "penicillin" can either refer to several variants of penicillin available, or to the group of antibiotics derived from the penicillins. Hetacillin has in vitro activity against gram-positive and gram-negative aerobic and anaerobic bacteria. The bactericidal activity of Hetacillin results from the inhibition of cell wall synthesis and is mediated through Hetacillin binding to penicillin binding proteins (PBPs). Hetacillin has been withdrawn from the market since it has been discovered that it has no therapeutic advantage compared to non-ester derivatives like ampicillin.