Carbenicillin was administered during 56 episodes of gram-negative bacilli infection occuring in patients with acute leukemia and metastatic cancer. The response rate was 91% in 23 episodes ofPscudomonassp. infections, and 58% in 12 episodes ofE. coliinfections. Carbenicillin was ineffective against infections caused bySerratiasp. andKlebsiellasp. The majority of patients who responded become afebrile within 48 hours. Toxicity was limited to nausea, vomiting, hypersensitivity reactions, and transient elevations in scrum transaminases.