['Grass group I consists of very potent allergenic components which are found in the pollen of all temperate grasses. Several post-translational modifications are predicted from the cDNA data.', 'The aim of this study was to identify sequential IgE-binding sites on the allergen Phl p 1 and to determine their influence on IgE reactivity.', 'Based on cDNA data and microsequencing results we synthesized overlapping decapeptides covering the complete Phl p 1 molecule and tested them for immunological reactivity by means of the PEPSCAN technique. In a dot test we determined the frequency of IgE reactivities to post-translationally modified structures (hydroxylated proline residues, carbohydrate structure, and disulphide formations).', 'Screening by overlapping peptides demonstrated an IgE binding site on the 10 N-terminal amino acids. Comprehensive studies showed that the two hydroxyproline residues of the native Phl p 1 allergen (at positions 5 and 8) and the N-glycan (at position 9) can result in an increased IgE reactivity; 3.3% of the sera exclusively bound to the hydroxyproline bearing peptide, while only 0.4% bound to the proline containing peptide. With regard to glycosylation, we estimated that 20% of sera recognized protein and carbohydrate epitopes, while one serum exclusively bound to the glycan. The formation of disulphide bonds has no detectable effect on the IgE reactivity to Phl p 1.'] Do post-translational modifications influence IgE reactivity to the major allergen Phl p 1 of timothy grass pollen?