Azlocillin is a new semisynthetic penicillin with a broad spectrum of antibacterial activity. Azlocillin is highly active against Pseudomonas aeruginosa including many strains that are resistant to carbenicillin and ticarcillin. Unlike many other penicillins with activity against Enterobacteriaceae and Pseudomonas, azlocillin retains a high degree of potency against Gram-positive organisms, fastidious Gram-negative organisms, and many obligate anaerobes. Because azlocillin is susceptible to certain beta-lactamases it is not highly active against bacteria that are resistant to other penicillins and/or cephalosporins due to the production of such enzymes. Like other penicillins, azlocillin is bactericidal for susceptible strains. It also interacts synergistically with aminoglycosides against enterococci, certain Enterobacteriaceae, and Ps. aeruginosa.