Regular inhaled β₂ agonist causes tolerance to the acute protective effect of β₂ agonist against bronchoconstriction induced by chemical stimuli such as AMP, histamine, and methacholine. We examined a more clinically relevant stimulus, inhaled allergen, in a double-blind, cross-over, random-order trial in 13 mild atopic asthmatics, who had not used β₂ agonist for at least 4 weeks. We compared regular inhaled salbutamol (200 μg four times daily for 2 weeks) with placebo (2 weeks) for effects on bronchodilator response, baseline methacholine, and allergen airway responsiveness, and on the acute protective effect of salbutamol against both stimuli. Baseline forced expiratory volume in 1 s (FEV₁), bronchodilator response, and methacholine responsiveness were the same during both treatment periods. After regular salbutamol, the allergen PC₂₀ (provocation concentration producing a 20% FEV₁ decrease) fell by 0·91 (SD 0·66) (p=0·0009) doubling doses, and the protective effects of salbutamol on methacholine and allergen were both significantly reduced (p=0·026 and 0·025, respectively). Taking into account the reduced baseline allergen PC₂₀, the post-salbutamol allergen PC₂₀ was almost 2 doubling doses (1 94 [1·43], p<0·01) lower during salbutamol treatment. Thus, 2 weeks of regular inhaled salbutamol increased airway responsiveness to allergen but not to methacholine, and caused tolerance to the protective effect of salbutamol on bronchoconstriction induced by both stimuli. These effects of inhaled β₂ agonist provide further evidence to support detrimental effects of their regular use.