Allergic rhinitis is an increasing health problem and pollen allergens are amongst the main elicitors of hay fever symptoms. Allergenic pollen contains a set of differently allergenic proteins which are thought to play a role in the pollen germination and fertilisation process. They are released upon contact with the stigma or mucosa or upon pollen grain rupture. Although the determinants of allergenicity of these proteins are still largely undiscovered, accessibility and solubility are now thought to mainly influence allergenic potency. Pollen of 61 allergenic plants was investigated with scanning and transmission electron microscopy. Most of the minor allergenic plants like species of the families Salicaceae, Fagaceae or Ulmaceae show the typical pollen wall organisation with intine, compact endexine and ektexine whereas in the majority of the major allergenic plants like species of Betulaceae and Poaceae the endexine is not detectable. Ambrosia artemisiifolia exhibits a laminated endexine. In addition, pollen of these major allergenic plants does not have electron-dense pollenkitt and starch is stored in a high proportion of the examined pollen. The question is raised whether pollen morphology and ultrastructure might contribute to the accessibility and therefore allergenicity of allergenic proteins.