To investigate the relationship of the major allergens of birch (Bet v I), alder (Aln g I), hazel (Cor a I), and hornbeam (Car b I) at the nucleic acid level, a cDNA clone coding for the complete Bet v I protein was used for Northern and Southern blot experiments. RNAs were isolated from pollen of birch (Betula verrucosa), alder (Alnus glutinosa), hazel (Corylus avellana), and hornbeam (Carpinus betulus). Hybridization was performed at different stringencies. At high stringency, comparable binding of the complete Bet v I cDNA probe to pollen RNAs from birch, alder, and hazel could be observed, indicating high homology of the mRNAs coding for these allergens. With the 3' and 5' half fragments of the Bet v I cDNA, both probes bound to transcripts of all four tree pollens, but most strongly to birch RNA. In Southern blots, distinct binding patterns of genomic DNA digests of birch, alder, hazel, and hornbeam were observed. Most bands were observed with birch DNA digests and less with alder, whereas in genomic DNA digests of hornbeam and hazel, only one band was observed. The result of these cross-hybridization experiments indicate a high homology at the nucleic acid level of the four major allergens of trees belonging to the order Fagales. The sequence similarity presented here further corroborates earlier observations of immunologic cross-reactivity at the protein level. Therefore, in the case of the major allergens of the Betulaceae, an extract with only one major allergen, preferentially Bet v I, instead of all four major allergens, should be sufficient for diagnostic and therapeutic purposes.