The major allergen Phlp 5b from timothy grass pollen induces allergic rhinitis and bronchial asthma in millions of allergic patients worldwide. As an important step towards understanding the interactions between the pollen protein and components of the human immune system, the structure of the C-terminal key domain of Phlp 5b has been determined at 2.0 A resolution and refined to an R value of 19.7%. This is the first known allergen composed entirely of alpha-helices. The protein forms a dimer stabilized by one intermolecular disulfide bridge. Sequence homology suggests that at least all group V and group VI grass-pollen allergens belong to this new class of 'four-helix-bundle allergens'.