The discovery of diphtheria toxin from culture medium of Corynehacterium diphtheriae was one of the greatest discoveries in medical history. Since this epoch-making change in medicine, diphtheria toxin has been studied, first to prevent disease and later to understand the pathogenic mechanism at the molecular level. A number of important discoveries in the fields of bacterial pathogenesis and bacterial protein toxins have resulted from these studies, such as: (1) antibodies directed against the toxin can protect against the disease, (2) diphtheria toxin acts within cells by inactivating a target protein by adenosine diphosphate (ADP)—ribosylation, and (3) the toxin consists of two segments with distinct functions. Because of extensive studies over a long period, diphtheria toxin is one of the best known of the bacterial protein toxins, and discoveries, such as those enumerated above, have frequently led to increased understanding of the biology of other bacterial toxins.