MYCOBACILLIN, a new antifungal antibiotic isolated from the culture filtrate of a strain of Bacillus subtilis, is a cyclic peptide containing 13 residues of 7 different amino-acids, the sequence of which has also been worked out1-2. Now the stereotypic nature of the constituent amino-acids is an interesting query. Antibiotics are often characterized by the presence of D-amino-acids. Penicillin and antibiotics of actinomycin group contain D-valine3. Actinomycin also contains D-alloisoleucine. Likewise, D-phenylalanine is the constituent of gramicidine S4 and tyrocidines5. Extreme examples are, however, polymyxin-B, polymyxin-D and bacitracin A. Polymyxins contain D-phenylalanine, D-leucine, D-serine and D-α γ-diaminobutyric acid, and bacitracin contains D-ornithine, D-glutamic acid, DL-aspartic acid and D-phenylalanine6. This communication records the stereotypic nature of each of the constituent amino-acids of mycobacillin. In essence the method is based on the substrate specificity of D- and L-amino-acid oxidases and L-glutamic acid dehydrogenase.