Ph. Eur. Mixture of the sulfates of polypeptides produced by the growth of certain strains of Bacillus polymyxa, the main component being Polymyxin B1.Assay (HPLC): sum of Polymyxins B1, B2, B3 and B1-I min. 80 %; Polymyxin B3 max. 6 %; Polymyxin B1-I max. 15 % (all data based on dried substance).Peptide antibiotic that mainly acts against Gram-negative bacteria. Causes changes in membrane structure resulting in leakage of small molecules.