The secretion layer over the bronchial epithelium was investigated on human biopsy material. In the sol phase of the secretion, phospholipid membranes stretched out or arranged in vesicular structures could regularly be demonstrated. They displayed the same structure as the surfactant material in the alveolar space. It can be inferred therefrom that the surfactant is excreted via the bronchial system. It probably contributes to the stabilization of the sol phase of the mucus and thus influences the sliding capacity of the gel phase. So the surfactant has an important function in maintaining mucociliary clearance at the bronchial mucosa.