Certain of the cyclic cationic polypeptide anti-biotics, polymyxin B, colistin, and tyrocidine, prevent the lethaleffect of endotoxin for chick embryos when admixed with the toxin prior to intravenous injection. In this system, 25, ug of polymyxin B sulfate prevented the toxicity of 0.217, ug (241 LD50) of Escherichia coli endotoxin; an antibiotic-endotoxin weight ratio of 115: 1.(D. Rifkind and JD Palmer, J. Bacteriol. 92: 815, 1966). The present study was undertaken to determine whether polymyxin B will also prevent endotoxin lethality in a mammalian host system.

E. coli 0111: B4 Westphal endotoxin (Difco), alone or mixed with polymyxin B sulfate (Bur-roughs Wellcome & Co., Tuckahoe, NY), was injected intravenously, in a final volume of 0.2 ml in pyrogen-free saline, into 8-to 10-week-old female Swiss-Webster mice (Simonsen Laboratories, White Bear Lake, Minn.). Lethality was scored at 48 hr, and the LD50 was estimated by the Reed-Muench method (L. J. Reed and H. Muench, Am. J. Hyg. 27: 493, 1938). A 50-, ug amount of polymyxin B was used, as this dose produced no lethality (0 of 20), whereas 100, ug resulted in a 33%(23 of 70) mortality. The LD50 of endotoxin alone was 36 MAg, and this was not significantly decreased by the admixture of 50 Mug of polymyxin B (21 Mug, Table 1). The antibiotic-endotoxin ratio which could be tested in the Swiss-Webster mouse system, was, however, relatively small as compared with that which provides protection in chick embryos (see above). As the mice were relatively sensitive to polymyxin B, the antibiotic-endotoxin ratio could be increased onlyby decreasing the resistance of mice to endotoxin. This was accomplished by two methods, the first of which was blockade of the reticuloendothelial system with lead acetate (H. Selye, B. Tuchweber, and L. Bertok. J. Bacteriol. 91: 884, 1966). A 1-mg amount of lead acetate, Pb (C2H302) 2. 3H20,(Fisher Scientific