The RAST value obtained in the A. fumigalus system, from sera of patients with allergic bronchopulmonary aspergillosis (ABPA) is dependent not only on the reagin titre but also on blocking factors that depress the RAST score. These factors are primarily specific for A. fumigatus and can be removed by an A. fumigatus immuno-adsorbent. However, some blocking activity could be found in normal sera even after absorption with an A, fumigates immunoadsorbent and seems to be non-specific. All of these blocking factors influence the specificity of the RAST, and complicate the comparison of the quantitation of IgE antibodies in different sera. The use of discs rather than cellulose particles and the use of dilute sera result in a RAST assay more proportional to the specific IgE antibodies to A. fumigates.