A monoclonal antibody assay has been developed to measure Der p I specific IgE in sera of D. pteronyssinus sensitive patients. In this assay a specific monoclonal antibody was bound to the solid-phase and this complex was used for insolubilization of the allergen. Two procedures using two different solid-phases, CNBr activated paper discs and microtitre plate wells, were compared. In the paper disc assay about 90% of specific IgE was bound to the solid-phase. A study of 80 sera from mite sensitive children confirmed the importance of Der p I; indeed all the sera contained Der p I specific IgE and IgE anti Der p I contributed from 5% to 100% (mean = 39%) of the mite specific IgE response. In the microtitre plate assay only 45% of specific IgE was immobilized and it was necessary to express the results in arbitrary units. The correlation with the paper disc assay was significantly positive (r = 0.89) but five samples were found to be negative. However, this assay appears to be of interest for studying the affinity of specific IgE in different samples. The use of specific monoclonal antibodies as allergosorbents is a useful approach to a better standardization of the in vitro diagnostic reagents.