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Asthma can be considered a combination of three features: airways obstruction with spontaneous and pharmacologic reversibility, increased airways responsiveness to exogenous and endogenous stimuli, and airways inflammation. Airways hyperresponsiveness, usually measured in the laboratory as the provocative concentration or dose of a bronchoconstrictor substance causing a 20070 drop in FEY 1 (PC20 or PD20), broadly correlates with clinical indices of disease activity such as treatment requirements (1), diurnal variation of peak expiratory flows (PEF)(2), and airway responses to exercise (3) and inhalation of cold air (4). Increased airways responsiveness is also a feature of other inflammatory diseases of the lungs (5-7), including acute respiratory infections (8). It is also demonstrable in the experimental setting such as that of ozone-induced bronchial inflammation (9) and can therefore be as-