Butirosin is a new aminoglycosidic antibiotic complex which has broad gram-negative and gram-positive inhibitory antibacterial activity, as well as some bactericidal properties. Significantly susceptible bacteria include strains of Staphylococcus aureus and Streptococcus pyogenes, and pathogenic gram-negative species such as Enterobacter aerogenes, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis and P. vulgaris, Salmonella enteritidis and S. typhimurium, Shigella flexneri and S. sonnei. Good activity by parenteral dosing was obtained in various acute mouse infections. Butirosin is especially interesting because of its activity against Pseudomonas aeruginosa in vitro, including gentamicin-resistant clinical isolates, and in experimental mouse infections at relatively low doses.