Recent evidence suggests that the surface fluid secreted by human airway epithelial cells contains antimicrobial activity. The identification of the factor(s) responsible for the antibacterial activity in airway surface fluid may bring new insights into lung defense mechanisms and the pathogenesis of disease. Defensins are small cationic peptides with broad-spectrum antimicrobial activity, and are therefore candidates for the bacterial killing factor in airway surface fluid. They are produced in mucosal epithelia and neutrophils of several species. To date no defensins have been isolated from human airway epithelia; however, such peptides are produced by bovine airway epithelia. A cDNA fragment for a human beta-defensin-1 (hBD-1) was recently cloned from the kidney. We cloned the full-length coding sequence for hBD-1 from human airway epithelia. The amino acid sequence for hBD-1 shared homology with the signal/propiece and mature peptides of other beta-defensins, including bovine tracheal antimicrobial peptide and lingual antimicrobial peptide. By Northern analysis, a single approximately 400-nt hBD-1 transcript was expressed in airway epithelia. Ribonuclease protection analysis demonstrated that hBD-1 transcripts were more abundant in the conducting airways than the gas exchange regions of human lung and that the expression was developmentally regulated. Human beta-defensin-1 may contribute to the antimicrobial activity of airway surface fluid and play a role in the mucosal defenses of the lung.