The concept of chemotherapy is almost as old as the science of bacteriology. Attempts were made in the late nineteenth century to treat infections such as tuberculosis by the injection of other organisms or their products. The property of one micro-organism to interfere with the growth of another is called anti-biosis, this action being frequently due to specific diffusible metabolic products of bacteria and fungi; these substances were therefore called antibiotics. The first to be clinically effective was tyrothricin, a product of Bacillus brevis, and a mixture of gramicidin and tyrocidine. Unfortunately these antibiotics are relatively toxic to leucocytes and are of value only for topical application.