Platelet activating factor (PAF), a phospholipid inflammatory mediator, was given as an aerosol to eight normal subjects. PAF caused a dose-dependent bronchoconstriction in all subjects. This did not correlate well with responsiveness to methacholine. Some subjects showed tachyphylaxis to PAF-induced bronchoconstriction. No subject had a late bronchoconstriction response. Transient facial flushing and an increase in heart rate (mean 7 beats/min) occurred but there was no consistent change in blood pressure. Lyso-PAF, the inactive precursor and major metabolite of PAF, had no effect on pulmonary or cardiovascular responses. Six of the subjects took part in a double-blind, randomised, placebo-controlled, crossover study in which bronchial responsiveness to methacholine was measured over the 3 days after administration of PAF or lyso-PAF. PAF had a greater effect in raising responsiveness (p less than 0.01). Its maximum effect occurred at 3 days and returned to baseline in 1 to 4 weeks. PAF may contribute to the pathogenesis of bronchial hyperresponsiveness, which is the most characteristic abnormality in asthma.