Mucosal surfaces of several organ systems are important interfaces for host defense against microbes. Recent evidence suggests that antimicrobial peptides contribute to the defense of these surfaces. Defensins are one family of antimicrobial peptide, but their known distribution in humans has been limited to four members found in cells of myeloid origin. We sought to determine if the human defensin family was more complex. We found that the family of human defensins is diverse and is not restricted to expression in leukocytes. Southern blot and genomic clone analyses reveal that numerous defensin-related sequences are present in the human genome. A gene for a new human defensin family member was characterized. This gene, designated human defensin-5, is highly expressed in Paneth cells of the small intestine. This is the first example of an antimicrobial peptide gene expressed in an epithelial cell in humans. The data support the hypotheses that epithelial defensins equip the human small bowel with a previously unrecognized defensive capability which would augment other antimicrobial defenses.