This study is designed as a two centre, randomised, placebo-controlled, escalating single dose study in up to 88 cat allergic subjects. Cohorts of 8 subjects will be enrolled. Each cohort will undergo screening 14-28 days before treatment and a baseline challenge for EPSR and LPSR to cat allergen injected into the arm 7 days before treatment. On the treatment day, subjects will be injected either intradermally (into the skin) or subcutaneously (under the skin) with a single dose of Cat-PAD or placebo and safety observations made for 8 h. After 21 days, subjects will again have cat allergen injected into the arm and the EPSR and LPSR will be recorded. The dose of Cat-PAD will be increased in successive cohorts, provided that the previous dose tested was well tolerated.