Three isoforms of the major birch pollen allergen, Bet v, 1 from Betula verrucosa have been expressed as recombinant proteins in E. coli and purified. The immunochemical properties of recombinant isoforms (rBet v 1) differed on immunoblots when compared using Mabs and birch pollen allergic patients serum IgE. 2-D gel analysis showed that recombinant isoforms with different epitope structure can focus under the same protein spot after electrophoresis. The structure of conformational epitopes can be distorted by amino acid substitutions even when T-cell epitopes are not affected as judged by T-cell proliferation studies.