The effects of intramuscular injections of minute amounts of polymyxin B were studied in 42 patients with endotoxemia. Plasma endotoxin was measured by means of an endotoxin-specific Endospecy test (Seikagaku Corp., Tokyo, Japan) after pretreatment of the plasma with a new perchloric acid method that we developed. The normal value of plasma endotoxin is less than 9.8 pg/mL. Polymyxin B was administered at a dose of 12,500 U every 6 hours. Plasma endotoxin rapidly decreased to the normal range in 40 of the 42 patients. Body temperature fell significantly. APACHE II scores were also significantly improved. Tumor necrosis factor-alpha and interleukin-6 (IL-6) decreased in survivors, while tending to persist in high values in patients who died. No side effects were observed in any of the patients. In conclusion, intramuscular injections of small doses of polymyxin B were useful in the treatment of endotoxemia.