The epithelial surfaces of the urinary tract, like those of other mucosal surfaces, are covered by a thin layer of fluid secreted by the epithelial cells and any inflammatory cells that may have been recruited in response to infection or inflammatory stimuli. As first described for nasal secretions by Alexander Fleming in 1922, epithelial secretions possess antimicrobial properties that are due mostly to their content of various antimicrobial proteins and peptides. These substances are present constitutively, but their activity can be augmented by the release of additional substances from epithelial cells and phagocytes stimulated by infection or inflammation. Among the antimicrobial substances identified in various epithelial secretions, lysozyme, lactoferrin, secretory leukoprotease inhibitor, peroxidases, and defensins are the most abundant. It is noteworthy that many of these substances are also prominent components of the microbicidal granules of polymorphonuclear leukocytes [1]. The various antimicrobial substances act by distinct mechanisms, and there is increasing evidence that they often act synergistically.