OUR knowledge of the effects of antibacterial substances produced by or derived from bacteria or fungi continues to increase. A new antibacterial substance named 'bacitracin' has been described by B. A. Johnson, H. Anker and F. L. Meleney (Science, 102, 376, 1945 ; and see the Lancet, 641, Nov. 17, 1945). It is produced by a Gram-positive aerobic sporing rod, and the crude brew obtained inhibited a haemo-lytic streptococcus when it was diluted 1: 2,000 to 1: 10,000. Some of the most susceptible organisms were 3-hæmolytic streptococcus, pneumococcus, some clostridia, anaerobic staphylococci and some strains of anaerobic streptococci. Gram-negative organisms were not sensitive to it. 'Bacitracin' is soluble in water and butanol, unstable to alkali, stable to acid and to heating to 100° C. for fifteen minutes. Impure extracts were not toxic to animals and did not irritate the conjunctiva. A single intraperitoneal injection of it ensured the survival of 80 per cent of mice infected via the peritoneal cavity with a streptococcus, but 30-40 per cent also survived after a single subcutaneous injection and 80-90 per cent after subcutaneous injections repeated every three hours. Subcutaneous injections repeated every three hours ensured the survival of 80 per cent of guinea pigs infected with the gas-gangrene organisms, Clostridium welchii and C. septicum. It is claimed that local treatment of human infections caused by susceptible organisms are comparable to those of penicillin.