We compared the expression of human ~-and ~-defensins by various human tissues, mRNA for cc-defensins HNP1-3, abundant in bone marrow, was detected in peripheral blood leukocytes, spleen and thymus by RT-PCR, which revealed cc-defensins HD5 and HD6 only in the small intestine. In contrast, the pancreas and kidney expressed high levels of hBD-1 and lower levels of this [~-defensin were found in many organs by RT-PCR (salivary gland > trachea > prostate and placenta > thymus, testis, small intestine), hBD-1 mRNA was produced constitutively by cultured normal human epithelial cells derived from the trachea, bronchi, small airways and the mammary gland. These largely non-overlapping tissue distributions of human a~-and [~-defensins suggest that hBD-1 may be positioned to defend epithelial cells and mucosae from infection, whereas expression of HNP1-3 in neutrophils and HD5 and HD6 in Paneth cells allows these cc-defensins to participate in systemic and small intestinal host defenses, respectively.