Fifty-three patients were investigated by intradermal tests and Phadebas RAST and, in most cases, the serum IgE level was determined. They were tested for sensitivity to house dust, Dermatophagoides pteronyssinus, cat, dog, horse and mould allergens. All cases with IgE antibodies to house dust were found to have specific IgE to other tested allergens as well. Eighty-nine per cent of the patients being RAST positive for house dust were also positive for D. pteronyssinus and in the majority of those cases IgE antibodies to other allergens were detected. The results reaffirm previous findings that the mite is not the only allergen in house dust and that extract of house dust is a mixture of several allergenic substances, which make it less suitable for differential testing. It is concluded that serum IgE determination is of value as a screening procedure in the diagnosis and that the use of a number of specific allergens in RAST is valuable in pinpointing the offending allergen in house dust hypersensitivity.