Bacillomycin S, an antifungal antibiotic from culture filtrates of Bacillus subtilis, was shown to consist of proteinaceous matter (83.6 g/100 g) and lipid material (16 g/100 g). The amino acid and fatty acid components were determined qualitatively and quantitatively. It was shown that bacillomycin S is stable at physiological pH and it is suggested that its antifungal activity resides in the intact molecule.