The objective of this study is to compare the relative bronchoprotection (at trough) conferred by 2-week chronic dosing with levosalbutamol and racemic salbutamol in mild to moderate asthmatics. Patients will be preselected into two groups on the basis of their beta-2 adrenoreceptor polymorphisms. The investigators will evaluate if this has a differential effect on the bronchoprotection conferred by both formulations of salbutamol, as evidenced by a rebound in airway hyper-responsiveness, in order to determine whether any effect can be explained by the S enantiomer in the racemic formulation.