EM49, a novel peptide antibiotic produced by Bacillus circulans, shows broad-spectrum antibacterial activity in vitro and also has substantial activity against yeasts, fungi, and protozoa. Its high degree of antipseudomonal activity and its greater activity against gram-negative than against gram-positive bacteria are noteworthy. EM49 is rapidly biocidal to populations of Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and Candida albicans. Even after ten successive subcultures in the presence of EM49, strains of P. aeruginosa, E. coli and S. aureus did not develop any resistance to this compound. The antibiotic was active in vivo upon subcutaneous administration to mice infected with Streptococcus pyogenes C203 or E. coli, but was inactive when given by the oral route. When applied topically at a concentration of 0.5 % in a cream base, EM49 prevented the multiplication of P. aeruginosa in experimentally induced wounds in mice.