Carbenicillin, a semisynthetic derivative of penicillin, was found to be active in vitro against many gram-negative bacterial species, including most Pseudomonas, Escherichia coli, Proteus, Salmonella, Haemophilus, and Serratia strains. Most Klebsiella-Enterobacter organisms and enterococci were quite resistant. During in vitro studies, serum had no effect on the minimal inhibitory concentration of this drug, whereas increased inoculum size required greater concentrations for inhibition. Resistance was found to develop rapidly in vitro when Pseudomonas was employed. Pharmacological studies revealed peak serum levels of 9μg/ml one hour after parenteral administration of 500 mg of carbenicillin. Sixty percent of this dose was recovered in the urine within eight hours, producing concentrations of 500μg to 1,450μg per milliliter.