Allergenic cross-reactivity between members of the Fagales family (birch, alder, hazel, and beech) and between members of the Oleaceae family (ash, olive, lilac, and privet) is well known, but little is known about possible cross-reactivity between these two groups of trees, in particular between birch and ash, both of which flower in the spring. Various immunochemical methods including RAST inhibition, Western blot, and Western blot inhibition have been used in this study to show that there is partial cross-reactivity between birch and ash pollens. Enzyme allergosorbent test measurements were performed on sera from 35 patients with hay fever in spring by using birch and ash pollen allergen disks. The major allergen of birch, Bet v 1, was readily detectable in the birch pollen extract, but a homologous allergen in the ash pollen extract was barely detectable. Common allergens could be determined in the high molecular weight region. Ash pollen should be included in diagnostic procedures for spring pollinosis and should be considered for use in specific immunotherapy.