Leukocytes use an array of antimicrobial peptides and proteins to help them destroy invading microorganisms. These endogenous antibiotic molecules are remarkable for their structural variety, rapid evolutionary divergence, intraspecies variation, and complex yet subtle interactions with their targets. This arsenal has been studied most extensively in neutrophils, where its members include lactoferrin, secretory phospholipase A2, lysozyme, and the cathelicidins in the secretory granule compartment; defensins, bactericidal permeability inducing protein, serprocidins, and (again) lysozyme in the azurophil granule compartment; and calprotectin in the cytosolic compartment. That antimicrobial peptides and proteins are not limited to neutrophils and other phagocytes was made clear by the recent discovery of a microbicidal protein, NK lysin, in porcine natural killer cells and cytotoxic T lymphocytes. The structural homology of NK-lysin to amebapore, an antimicrobial cytolytic peptide of the parasitic protozoan Entamoeba histolytica, provides remarkable support for a long-suspected evolutionary connection between the leukocytes of higher animals and their unicellular, protozoan ancestors.