A new antibiotic, azirinomycin, has been discovered in the culture broth of newly isolated strains of actinomycetes. The original isolate from soil found to produce it has been identified as a strain of Streptomyces aureus. It was produced by submerged culture in shaken Erlenmeyer flasks in complex organic media. Azirinomycin and its methyl ester were found to exhibit broad spectrum antibiotic activity, in vitro, against both Gram-positive and Gram-negative bacteria; azirinomycin exhibits activity against both in agar diffusion assays. Both azirinomycin and its methyl ester were toxic to mice, and failed to protect them against lethal bacterial infections. This report will present biological data relevant to a description of the antibiotic, its preparation by fermentation, and the nature of producing microorganisms. Purification and chemical identification of azirinomycin as 3-methyl-2-(2H)azirinecarboxylic acid are reported by Miller, et al. Although the antibiotic is unstable in solution, especially when purified, it was possible to determine biological activities of freshly prepared solutions. Biological characteristics were also determined for the more stable, chemically-prepared, methyl ester.