The polymyxins are a group of basic peptides active against gram-negative bacteria and include polymyxin B and polymyxin E(colistin). Polymyxins act as cationic detergents. They attach toand disrupt bacterial cell membranes. They also bind and inacti-vate endotoxin. Gram-positive organisms, Proteus sp, and Neisseria sp are resistant.Owing to their significant toxicity with systemic administra-tion, polymyxins are largely restricted to topical use. Ointments containing polymyxin B, 0.5 mg/g, in mixtures with bacitracin or neomycin (or both) are commonly applied to infected superficial skin lesions. Emergence of strains of Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae that are resistantto all other agents has renewed interest in polymyxins as a paren-teral agents for salvage therapy of infections caused by these organisms.