Purified tomato allergen has been subjected to several analytical procedures and both the UV absorption spectra and the fluorescence emission curves were studied. Spectroscopic analysis indicates the incorporation of N-glycosidic 1-amino-1-deoxy-2-ketose groups in the allergen molecules. The "available" lysine content of the atopic allergen is much lower than the total amount of lysine present in the digest after HC1 hydrolysis and in the active fraction browning intermediates may be demonstrated. The allergen is relative resistant to the action of trypsin and chymotrypsin. The tomato allergen is unstable in alkaline medium, the skin-reactive potency decreases markedly by milk oxidation with K3Fe(CN)6 at pH 5.6 and the allergen was artificially produced by non-enzymatic browning reactions during the ripening and the storage of tomato fruits. The structural centers involving the N-glycosidic protein-sugar linkage may be the characteristic allergenic determinants in the atopic allergen molecules.