Cloning of the gene encoding the major allergen, Car b I, from Carpinus betulus (hornbeam) pollen was performed using the Polymerase Chain Reaction (PCR) to specifically amplify the gene of interest using single stranded cDNA as template. Specific primers, deduced from the aminoterminal sequence of the purified protein, were tailored to facilitate direct expression of plasmic clones, and the large fraction of positive clones obtained, revealed the presence of isogenic variation. Three clones were characterized in detail by antibody based assays and nucleotide sequencing. The recombinant allergens were shown by crossed immunoelectrophoresis (CIE) to precipitate with monospecific polyclonal rabbit antibodies raised against purified Bet v I, by crossed radioimmunoelectrophoresis (CRIE) to bind tree pollen allergic patient serum IgE, and by immunoblotting to bind murine monoclonal antibodies, raised against purified Car b I from pollen. Car b I is encoded by a 159-triplets open reading frame. The molecular masses (M(r) = 17272, 17355 and 17217 Da, respectively), the amino acid composition, and the aminoterminal sequence of the predicted polypeptides agree well with data obtained by analysis of the protein purified from pollen. The deduced amino acid sequences show pronounced homology (73, 75 and 74% identities respectively) to Bet v I, the major allergen from Betula verrucosa (white birch) pollen. Soluble recombinant Car b I, without a fusion partner, was produced in Escherichia coli with an immunochemical reactivity closely resembling that of the native pollen allergen. The tree pollen major allergens therefore constitute an ideal system for the study of allergenic epitopes.