Previous studies showed that pollens from trees of the order Fagales (e. g. birch, alder, hazel and hornbeam) all contain one major allergen. These proteins are cross-reactive between these tree species, and approximately 95% of tree-pollen-allergic patients display IgE binding to these allergens. Using the reported N-terminal amino acid sequence of the hazel pollen allergen Cor a I, it was possible to amplify Cor-a-I cDNA by use of the polymerase chain reaction. Four clones with cDNA inserts were isolated. All four clones contained an open reading frame of 477 nucleotides (159 amino acids) but differed in length of their 3′-non-coding regions. Within the overlapping regions, the nucleotide sequence of the 3′-non-coding regions of the four clones were nearly identical. The open reading frames coded for different isoforms of the major hazel pollen allergen, Cor a I. The clones were designated Cor a I/5, 6, 11 and 16, respectively. Comparison of the deduced amino acid sequences of these Cor a I isoforms revealed identities of 96–99%. The sequence identities between the Cor a I isoforms and Bet v I, the major birch pollen allergen, were 71–73% (80.5–83% similarity). Comparing amino acid sequences of Cor a I isoforms with the published sequences of Aln g I, the major allergen from alder, and Cor a I and isoforms, the major allergen from hornbeam, 75.5–76.7% identity (83.6–85% similarity) and 83.6–89.9% sequence identity (89.3–95% similarity), respectively, was found. The four Cor a I cDNAs were subcloned into plasmid pKK223-3 and expressed in Escherichia coli as non-fusion proteins; their capacity to bind serum IgE from tree-pollen-allergic patients was investigated. The four cloned isoforms showed an apparent molecular mass of 17 kDa in SDS/PAGE, identical to the natural, pollen-derived Cor a I. IgE antibodies from tree-pollen-allergic patients was reacted with all four recombinant isoforms. However, we noted marked differences in the IgE-binding patterns of the distinct isoforms. Furthermore, Cor a I/11 was the only isoform recognized by the anti-(Bet v I) mAb, BIP 1. Our results demonstrate that Cor a I isoforms display different antigenic properties, very likely due to few but significant changes in their amino acid sequences. These findings have implications for the development of reagents for diagnosis and immunotherapy of type I allergies.