In May 1947, Benedict and Langlykke1reported that sterile culture filtrates of Bacillus polymyxa in dilutions of 1:1,000 inhibited the growth of Brucella bronchiseptica in culture. Shortly after this report, Stansly, Shepherd and White2independently described the isolation of an antibiotic substance from B. polymyxa which they named "polymyxin." This antibiotic was shown by them to be effective only against gram-negative organisms in vitro, and to possess a high degree of therapeutic activity against experimental infections produced in mice by the inoculation of Klebsiella pneumoniae or Pasteurella multocida. It was also effective against experimental fowl typhoid infections in chicks. Subsequently, Stansly and Schlosser3established the identity of polymyxin as a new antibiotic and developed a method for its bio-assay. Not long after the two original reports on this antibiotic appeared, Ainsworth, Brown and Brownlee4announced that they had isolated an antibiotic, active against gram-negative organisms, from