Proteins which destroy various organisms are widely distributed in nature. Some of these proteins exert their toxic effect by inhibiting protein synthesis in a specific manner. Thus, Colicin E 3 kills susceptible bacteria by inhibiting ribosome function. It does this by splitting off a small fragment from the t6S ribosomal RNA thus inactivating the 30S ribosomal subunit. Diphtheria toxin inhibits protein synthesis in eukaryotes by inactivating peptidyl-transferase II; this enzyme is necessary for the elongation of peptide chains on the ribosomes. Two toxic proteins from plants, abrin and ricin, also apparently interfere with some process necessary for the elongation of already initiated peptide chains. The possible physiological significance of the toxic proteins is discussed. There are in bacteria, as well as in plants and animals, proteins which have a highly toxic effect upon other organisms. Very small amounts of these toxic proteins, often termed "toxins" or "protein toxins", exert a pronounced effect and the lethal dose is often a few ~zg per kg body weight, or even less. Some cells and organs are highly sensitive to the action of a particular toxic protein, whereas others are virtually unaffected. Although their mechanism of action at the molecular level is largely unknown, it has recently become clear that some toxic proteins act by inhibiting protein synthesis.