The simultaneous presence of IgE reactivity to tomato fruit and grass pollen allergens is evident in many patients with allergy and may be caused by cross-reactivity. Using sera from polysensitized patients with a positive enzyme allergosorbent test (EAST) result (score >2), we tested reactivity to both allergen sources. IgE reactivity against both extracts was demonstrated in eight serum samples, and cross-reactivity was confirmed by the EAST inhibition assay. The structures responsible for this cross-reactivity were identified by Western blotting: five of the eight sera demonstrated a 16 kd protein in both extracts, which was identified as profilin. Additionally, seven of the eight sera showed IgE binding to epitopes on carbohydrate moieties, which contained ⍺1,3 fucosylations. To determine the allergens of tomato fruit extract, we performed two-dimensional polyacrylamide gel electrophoresis blotting. We were able to demonstrate one highly concentrated and about 20 weaker proteins possessing terminal fucose residues. These are similarly found in grass pollen extracts. It is therefore postulated that the cross-reactivity is affected by profilins and similar carbohydrate determinants. If carbohydrate structures can provoke IgE cross-reactivity between phylogenetically distant species, such structures may play an important role in sensitization and mediator release. The ubiquitous nature of the IgE-binding determinants was studied by additional EAST inhibition tests with tomato allergen disks and extract from birch pollen, mugwort pollen, apple, and celery, leading to significant inhibitions among all these allergen sources. Epitopes exclusive to grass pollen and tomato have not been detected. (J ALLERGY CLIN IMMUNOL 1996;98:805-15.)