To clarify the mode of action of an oral bacterial extract (OM-85 BV) on local airway immunity pre- and posttherapeutic washings from bronchoalveolar lavage (BAL) fluid of 28 adult patients with nonobstructive chronic bronchitis were analysed. In comparison to healthy controls, an elevation of total cell count due to an increased number of PMN leukocytes, and an impaired activity of the alveolar macrophages measured by the chemiluminescence response to opsonized zymosan was observed in patients with chronic bronchitis. After treatment with OM-85 BV, the BAL CD4+/CD8+ lymphocyte ratio and BAL interferon-γ levels were increased. The alveolar macrophage activity was normalized and the BAL IgA was regulated from a reduced or hyperelevated to a moderately increased level.