The antiendotoxin effect of polymyxin B was investigated in experimentally induced septicemia in rabbits. The Pasteurella multocida organisms were sensitive to the antibacterial action of penicillin but not to polymyxin B. Animals pre-treated with polymyxin showed positive blood cultures and significantly reduced plasma endotoxin levels (Limulus test) with normal white blood cell and platelet counts when analyzed 6 hr after the injection of live organisms. Polymyxin therapy given after the animals had established septicemia-endotoxemia reduced the plasma endotoxin levels and improved the survival, but had no effect on the leukopenia and thrombocytopenia. The best survival data were obtained in rabbits who were treated with both penicillin and polymyxin. The data suggest that polymyxin is effective in neutralizing the endotoxic effects from live organisms and that the timing and perhaps duration of the polymyxin treatment is of critical importance. In addition, a modified Limulus lysate method was developed which showed that quantitative plasma endotoxin determination could be made more sensitive by prior heating of the plasma to remove the natural inhibitors.