IN a previous communication1, from the analysis of a partial acid hydrolysate, the structure of polymyxin E1 was shown to be limited to two alternatives, the so-called 7 α and 8γ formulæ. By applying the enzyme method of hydrolysis as devised by Suzuki et al. in their determinations of the structures of polymyxin B1 (ref. 2) and colistin A (ref. 3), the problem has now been resolved. We have isolated the peptides and from the hydrolysate of polymyxin E1 with the enzyme Nagarse (supplied by Teikoku Chemical Industry Co., Ltd., Osaka, Japan). The presence in particular of the second peptide eliminates the 8γ variant, and the structure of polymyxin E1 is therefore concluded to be (I; R=(+) 6-methyloctanoyl, X = D-Leu) and is therefore identical with that of colistin A (refs. 3 and 4).