Streptomycin has certain important limitations, chief among which are its neurotoxic potentialities and the development of resistance among infectious organisms upon its prolonged administration, with resistance remaining the major limitation especially in tuberculosis treatment. In connection with studies on antibiotic production by microorganisms, thousands of Streptomyces cultures were isolated from soils and other natural substrates and tested for activity against bacteria, with recent emphasis on mycobacteria. In search of compounds effective against streptomycin-resistant bacteria, notably streptomycin-resistant strains of Mycobacterium tuberculosis, a new agent designated as neomycin was discovered. The organism producing neomycin was isolated from the soil and is somewhat related to Streptomyces fradiae. Neomycin is a basic, thermostable compound soluble in water and insoluble in organic solvents, most active at alkaline pH. It is active against numerous Gram-positive and Gram-negative bacteria, especially mycobacteria, but not against fungi. Neomycin preparations possess several desirable properties: activity against both streptomycin-sensitive and streptomycin-resistant bacteria; considerable activity against various forms of M. tuberculosis and other mycobacteria; limited or no toxicity to animals; and in vivo activity against various bacteria including Gram-positive, Gram-negative, and streptomycin-resistant organisms.