BACKGROUND A number of recombinant allergens are by now constituents of devices that can be routinely used for the detection of specific IgE. Therefore, the results of diagnostic procedures using conventional allergen extracts can be compared with those employing selected recombinant allergens. METHODS Thirty-four sera from patients allergic to birch pollen were tested with the standard t3-CAP and rBet v 1a- and rBet v 2-CAP. cDNA was prepared by RT-PCR using primers according to the N terminus of purified allergens. Expression cDNA libraries were screened with IgE from selected patients. RESULTS Twenty-four patients allergic to birch pollen showed the same RAST class with t3 as with rBet v 1a; 8 patients differed within 1 RAST class. In addition, 3 patients showed RAST class 3 with rBet v 2. Besides Bet v 1 and Bet v 2, 3 allergens from celery and avocado belonging to highly conserved protein families were cloned and sequenced. CONCLUSIONS rBet v 1a can be expected to represent an excellent tool for the diagnosis of patients allergic to birch pollen in Central, Northern, and Eastern Europe. Still, a much higher number of patients has to be tested. For their high degree of conservation, further protein families have to be identified to explain cross-reactivities of birch pollen allergens other than Bet v 1 and Bet v 2 with, e.g., allergens from vegetable food.