Cecropins are potent antibacterial proteins that constitute a main part of the cell-free immunity of insects. Cecropins are small proteins of about 35 amino acid residues active against both Gram-positive and Gram-negative bacteria. They seem to exert a lytic action on bacterial membranes. Cecropins isolated from insects other than Hyalophora cecropia (Cecropia moth) have been given various names; bactericidin, lepidopteran, sarcotoxin, etc. All of these peptides are structurally related.