Fruit allergy is frequently associated with birch pollinosis. The aim of this study was to investigate which kiwi allergens were involved in subjects allergic to fruit alone and in patients allergic to both fruit and birch pollen. Sera of nine patients (five with both kiwi and birch pollen allergy and four with isolated kiwi allergy) were studied by immunoblot of kiwi extract. Eight of the nine sera reacted with the 30 kDa protein. Furthermore, IgE-binding proteins were seen at approximately 23 kDa (detected by five sera), 43 kDa and 80 kDa (four sera), and > 80 kDa (two sera). One serum showed no IgE binding to any kiwi allergen. The 30 kDa is the major allergen in kiwi and was purified by anion-exchange chromatography and characterized by isoelectrofocusing and amino acid sequencing. The comparison of its partial amino acid sequence with data from the Swiss Protein Bank revealed that this protein is actinidine. The carbohydrate structures in kiwi and birch pollen extracts were investigated with seven lectins. On kiwi blot, Aleuria aurantia agglutinin showed strong reactivity (indicating fucose residues) to the components of 35 to 92 kDa, while concanavalin A (indicating mannose, glucose or N-acetylglucosamine residues) showed weak binding at 67 kDa. In contrast, strong binding of Galanthus nivalis agglutinin (indicating mannose residues) and concanavalin A was found on birch pollen blots. The presence of IgE against carbohydrate structures was determined by means of enzyme-linked immunosorbent assay (ELISA) after periodate treatment of kiwi extract. The IgE binding was reduced by periodate treatment of kiwi coated microtiter plates, but not by sera reacting exclusively with the 30 kDa protein. Furthermore, selected sera were treated with proteinase K-digested kiwi and birch pollen extracts as the sources of crossreactive carbohydrate determinants. In accordance with the results of sodium periodate treatment, significant levels of anti-cross-reactive carbohydrate determinant IgE were found in sera from patients allergic to both kiwi and birch pollen. Our results show that the major allergen for kiwi allergy is the 30 kDa protein and additionally that the cross-reaction between kiwi and birch pollen allergy is mainly due to carbohydrate moieties.