The stimulation of intrabronchial mast cells and macrophages by inhaled allergens play a major role in allergic bronchial asthma. Bronchial surfactant is proposed to change cell surface receptor response in the bronchial tree. In this investigation natural surfactant was found to reduce bronchial obstruction after intratracheal ovalbumin challenge in artificially ventilated guinea pigs sensitized to ovalbumin. After ovalbumin challenge tidal volume decreased to 59.2 +/- 14 % of the control value in surfactant treated animals. In solvent treated and untreated control animals, tidal volume decreased to 18.8 +/- 11% and 18.1 +/- 11% of the control value, respectively.