Several proteins and polypeptides of reptilian, amphibian, insect, and microbial origin share a common cytolytic property. However, these cytolysins fulfill different objectives. They provide offensive armament in the case of toxins, but defensive systems in the case of antibacterial peptides. The sequences of several nonenzymatic cytolysins and their analogues were compared to identify the structural requirements for cytolytic activity. These cytolysins, although isolated from phylogenetically unrelated organisms, possess the common sequence features of a cationic site flanked by a hydrophobic surface. The presence of such a region apparently confers the cytolytic activity of various cytolysins. The concept of a cytolytic region is strongly supported by the existence of several natural and synthetic analogues of cytolysins and by chemical modification studies of these cytolysins. This prediction provides a new focus for cytolysin research. The understanding of this structure-function relationship should facilitate the design, synthesis, and development of better antibacterial and anticancer peptides.