A new method for the measurement of allergen-specific IgD (as-IgD) was developed by modifying the ImmunoCAP assay (Pharmacia), and amplification of the signal with a goat anti-human/rabbit antigoat detection system. The assay was sensitive enough to measure as-IgD in serum samples. The specificity of the assay was examined using inhibition tests with excess corresponding and non-corresponding allergens. For the different allergens inhibition rates between 56% (house dust mite) and 88% (cat) could be achieved. Non-corresponding allergens did not inhibit the as-IgD binding. Total IgE and allergen-specific IgE (as-IgE) was measured using the ImmunoCAP system. Total IgD was measured using a sandwich ELISA. As-IgD was measured in serum samples from 51 atopic and 2.1 non-atopic subjects, and the correlation with as-IgE was examined. As-IgD was detected in both atopies and non-atopies but at higher levels in atopies. As-IgD against birch pollen and timothy pollen allergen was found to be increased in atopies with IgE directed against these allergens compared to atopies without IgE against these allergens (P <0.02 and P <0.03). As-IgD against birch pollen allergen was higher in atopies with IgE specific to this allergen than in non-atopies (P <0.02). In contrast to total IgE and total IgD, significant correlations were observed between as-IgD and as-IgE against timothy pollen (r= 0.34, P <0.04), birch pollen (r= 0.38, P <0.05) and cat dander allergen (r= 0.52, P <0.01). The observed correlations between as-IgD and IgE suggest that IgD and IgE may be similarly regulated, and thus the measurement of as-IgD may give further insight into the regulation of IgE.