Chemoprophylaxis of certain respiratory infections has been described. The benefit and cost of chemoprophylaxis of tuberculosis have been compared and indications outlined. While some protection against influenzal infection by drugs of the adamantane group has been demonstrated, chemoprophylaxis against respiratory virus infections is as yet unavailable or impractical. In diseases associated with impaired bronchial clearance the influence of long term chemoprophylaxis on the natural history of the disorders becomes less certain. Morbidity in chronic bronchitis and bronchiectasis may be reduced. In cystic fibrosis, however, although the prognosis has been undoubtedly improved by chemoprophylaxis, the effects of long term treatment of infection in the later stages are uncertain.