Ara h 1, a major peanut allergen, is known as a stable trimeric protein. Nevertheless, upon purification of native Ara h 1 from peanuts using only size exclusion chromatography, the allergen appeared to exist in an oligomeric structure, rather than as a trimeric structure. The oligomeric structure was independent of the salt concentration applied. Subjecting the allergen to anion exchange chromatography induced the allergen to dissociate into trimers. Ammonium sulfate precipitation did not bring about any structural changes, whereas exposing the allergen to hydrophobic interaction chromatography caused it to partly dissociate into trimers, with increasing amounts of trimers at higher ionic strengths. The (partial) dissociation into trimers led to a change in the tertiary structure of the monomeric subunits of the allergen, with the monomers in Ara h 1 oligomers having a more compact tertiary structure compared with the monomers in Ara h 1 trimers. As structural characteristics are important for a protein's allergenicity, this finding may imply a different allergenicity for Ara h 1 than previously described. © 2006 American Chemical Society.