Five immunogenic peptides from the major allergens of tree pollen extracts (residues 23–38) were synthesized by Merrifield solid-phase peptide synthesis. The primary structures of these peptides were deduced from the known N-terminal amino acid sequence of the major allergen of hazel, HIa, and birch, Bet v I. In a segment of 16 amino acids (23–38), 2 residues were the difference between birch and hazel. The selection of the peptides was on the basis of optimal hydropathicity. The segment 29–34 of these peptides was predicted to prefer a helical rather than β-sheet conformational state. The insertion of amino acid residues during the synthesis was confirmed by the analysis of amino acid composition prior to coupling the preceding residue. The linearities of the synthetic chains were assigned by the primary structure analysis of the entire chain by automatic Edman degradation. The synthetic peptides were purified by gel filtration chromatography and reversed-phase high-performance liquid chromatography, resulting in satisfactory homogeneity of the preparations. The immunogenicity of the peptides was tested in rabbits using conjugated peptide-bovine serum albumin through a carbodiimide spacer arm. The results indicated that all constituents of the conjugate could elicit an immune response. The peptides gave precipitation lines in crossed immunoelectrophoresis when using rabbit antibodies against the conjugated preparation. All the synthetic peptides and analogues from region 23–38 could inhibit, in dose-response patterns, the binding of specific IgE to the intact molecules. Similar results were shown for the shorter segment 25–34, suggesting that the C-terminal tetrapeptide did not contribute to the activity. The allergenicity of these peptides and their analogues is currently examined by Prausnitz-Küstner inhibition, histamine release and provocation tests.