Azlocillin is an acylampicillin antibiotic with an extended spectrum of activity and greater in vitro potency than the carboxy penicillins. Azlocillin is similar to mezlocillin and piperacillin. It demonstrates antibacterial activity against a broad spectrum of bacteria, including Pseudomonas aeruginosa and, in contrast to most cephalosporins, exhibits activity against enterococci. Three new broad-spectrum penicillins--azlocillin, mezlocillin, and piperacillin--will soon be available for clinical use. Azlocillin and piperacillin are more active than carbenicillin or ticarcillin against Pseudomonas aeruginosa. Piperacillin and mezlocillin demonstrate significant activity against the Enterobacteriaceae, including many strains of Klebsiella pneumoniae against which the older penicillins carbenicillin, ticarcillin, and ampicillin are ineffective.