This study is designed as a multicentre, randomised, double-blind, placebo-controlled study of 4 treatment regimens in up to 120 cat allergic subjects. A total of 24 subjects will be randomised into each treatment group. Each subject will undergo screening up to 4 weeks before treatment. Baseline challenge will consist of exposure to cat allergen for 3 hours in an EEC on 4 visits on successive days a week before the first administration of Cat-PAD or placebo. Treatment will be administered as intradermal injections into the flexor surface of alternate forearms. There are 5 treatment regimens involving administration of Cat-PAD or placebo. Post-treatment challenge (PTC) will consist of exposure to cat allergen for 3 hours in an EEC on 4 visits on successive days 18 weeks after the first administration of study medication.