The radioallergosorbent test (RAST) was applied for quantitative estimation of IgE antibodies to various common allergens. The results could be expressed in per mille of a reference serum containing IgE antibodies to birch allergen. Since the allergen extracts vary considerably both on a quantitative and qualitative basis, the results cannot, with few exceptions, be compared between different allergens. However, for each type of allergen the results can be compared between samples obtained from different individuals or at different times. Birch and dandelion allergen extracts were found to be potent and give rise to allergen excess in the systems used. A comparison between these two allergens showed that a group of allergic children had on the average approx. 15 times more circulating IgE antibodies to birch than to dandelion allergen.