If diphtheria is a disease caused by a microorganism, it is essential that three postulates be fulfilled. The fulfilment of these postulates is necessary in order to demonstrate strictly the parasitic nature of a disease: 1) The organism must be shown to be constantly present in characteristic form and arrangement in the diseased tissue. 2) The organism which, from its behaviour appears to be responsible for the disease, must be isolated and grown in pure culture. 3) The pure culture must be shown to induce the disease experimentally. An early statement of Koch's postulates.