Pirbenicillin is a broad-spectrum semisynthetic penicillin with activity against both gram-positive cocci and gram-negative bacilli. Pirbenicillin is less active than either carbenicillin or ticarcillin against Proteus spp. It is as active as carbenicillin against Serratia spp. and Enterobacter spp. Pirbenicillin demonstrated eight- and fourfold better minimal bactericidal concentration values towards Pseudomonas isolates than those of carbenicillin and ticarcillin, respectively. Pirbenicillin binds to and inactivates penicillin-binding proteins (PBPs) located on the inner membrane of the bacterial cell wall. The drug is somewhat more stable in serum than carbenicillin. Pirbenicillin does not appear to inactivate gentamicin as rapidly as carbenicillin.