Intravenous inoculation of 6.0 x 10(10)Pseudomonas aeruginosa organisms into rhesus monkeys 4 days after intratracheal inoculation of 2.5 mg of vincristine sulfate resulted in fatal sepsis in all of three untreated control monkeys. After intramuscular administration of either 2.5 mg of gentamicin or 50 mg of carbenicillin per kg per day, three of four monkeys in each group survived. When both antibiotics were given at the same dose but in separate sites, six of eight monkeys survived. Antibacterial activity of serum from infected monkeys or normal monkeys was not appreciably different when the two antibiotics were combined. Under the conditions of this study, there was no apparent difference in response of monkeys treated either with gentamicin or carbenicillin alone or with the combination of the two antibiotics.