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Calu-3 cells, a human lung carcinoma cell line with properties like serous cells of the upper airway, were used to develop an in vitro model for airway antibacterial activity. Calu-3 cell monolayers were cultured on permeable supports at an air–liquid interface. Apical surface fluid (ASF) was collected by washing; antibacterial activity was assayed by incubating ASF washings with bacteria for 18 h and counting surviving colony-forming units. ASF washings killed Escherichia coli and Pseudomonas aeruginosa. Antibacterial activity was salt sensitive and dependent on protein concentration. After washing, approximately 30 h were required before antibacterial activity recovered to its initial level. After culturing with topical corticosteroids (budesonide, triamcinolone, or beclomethasone, 0.1 μ g/ml for 48 h), ASF antibacterial activity was 4- to 10-fold greater than the ASF from control monolayers. The increase in …