The allergenic potency of different birch, Timothy and mugwort pollen extracts was determined by means of a direct RAST titration allergen assay. For birch and Timothy allergens, the results of skin and provocation tests did not confirm the results of the in vitro determinations of allergenicity. There was a poor correlation between the results of skin tests and the results of Phadebas RAST for determination of specific IgE to mugwort, whereas the correlation between skin tests and RAST for other allergens was excellent. It is concluded that direct RAST titration allergen assay is not adequate for all kinds of allergen preparations and that the Phadebas RAST for mugwort is less sensitive than the RAST for other allergens. The diagnostic efficacy of the different allergen preparations could not be evaluated.