This is a study investigating whether 14 days of dosing with GW274150 has a beneficial effect on a model of asthma type inflammation called the allergen-induced late asthmatic response. This will be compared with the response after treatment with a dummy (placebo) and a tablet treatment for asthma called Singulair (montelukast). Subjects in the study will receive all 3 treatments in a random order. The study is double-blind so subjects will not know which treatment they are taking at any given time.