Platelet-activating factor (PAF) is a phospholipid which plays a role as a mediator in inflammation. Recently, it has been implicated in the induction of bronchial hyperresponsiveness in man. In order to establish the effect of PAF on bronchial reactivity, 6 normal subjects without bronchial hyperresponsiveness inhaled 400 micrograms of PAF in 10 divided cumulative doses. All subjects felt a hot flush and a slight tracheal irritation after the inhalation of PAF. Forced expiratory flows (FEF) were measured between each inhalation of PAF and did not change significantly. Bronchial reactivity to methacholine (MCH up to a dose of 2 mg) was determined 1, 7, 14 and 21 days after inhalation of PAF. Forced expiratory volume in 1 s (FEV1), forced expiratory flow between 25 and 75%, and at 75% of vital capacity (FEF25-75% and FEF75%) measured after the inhalation of 2 mg of MCH did not differ significantly from baseline values determined before PAF challenge. In conclusion, the administration of PAF by inhalation in tolerable doses does not induce bronchial hyperresponsiveness as determined by a reduction of 20% of FEV1 nor by more sensitive indicators of ventilatory obstruction, such as FEF25-75% and FEF75%.