The tree pollen major allergens (Aln g I), Bet v I, Car b I and Que a I, were purified to near homogeneity. Rabbit antibodies raised towards the major allergen Bet v I react with the major allergens of alder, hazel, hornbeam and oak, although with distinct variation in affinity. Immunochemically the major allergens seem to resemble one another more within than between the botanically established families of the fagale order. The physico-chemical parameters (molecular weight, pI values, amino acid composition and NH2-terminal amino acid sequence) of the major allergens exhibit a higher degree of consistency than do the immunochemical parameters, indicating that nuances in the allergens' secondary and tertiary structures are likely to govern the immunochemical differences.