To examine the effects of acid exposure with moderate acidity (pH 3.0-5.0) on bactericidal activity of airway surface liquid (ASL), ASL was collected by washing the surface of primary cultures of human tracheal epithelial cells 24 h after treatment with phosphate-buffered saline (PBS) adjusted to a pH of 3.0, 4.0, or 5.0. In all ASL, bactericidal activity was sensitive to sodium concentration. Escherichia coli (500 colony forming units [CFU]) was incubated in ASL, and the number of surviving bacteria was examined. The number of surviving bacteria in ASL from cultured cells with acid exposure at pH 3.0-5.0 was significantly higher than that in control ASL. The minimum inhibitory dilution ratio of ASL against 500 CFU of E. coli was also examined by microdilution assays. According to this assay, the bactericidal activity in ASL with acid challenge at a pH of 3.0 was less than half of that in control ASL. Reverse transcription-polymerase chain reaction and Western blot analysis showed that the production of mRNA and protein of human beta-defensin (HBD)-1 were significantly decreased by acid exposure at pH 3.0-5.0. In contrast, acid exposure did not change the production of mRNA and protein of HBD-2 and beta-actin mRNA. These results indicate that acid exposure, even with moderate acidity, may inhibit the production of bactericidal molecules, including HBD-1, in airway epithelial cells. Acid exposure may reduce bactericidal activity of ASL in human airway epithelial cells and may increase susceptibility of the airway to bacterial infection.