In recent years chemotherapy has become increasingly important in all branches of medicine. The advent of antibiotics has provided a means by which many of the infectious diseases can be treated with remarkable success. Continued progress with synthetic chemicals and antibiotics is to be expected in the next few years. Such progress is dependent upon a more fundamental knowledge of the mode of action of these chemotherapeutic agents. In the past most research has been devoted to the discovery of new agents without much thought as to their mode of action. The method of "screening" new compounds for their beneficial effects has yielded remarkable results. However, the time has come when as an important adjunct to this type of research the fundamental aspect of the mode of action must accompany the former method.