Dermatophagoides farinae extract was filtered through Sephadex G-50 and DEAE-cellulose columns to study its chemical composition. Fractionation on Sephadex G-50 resulted in one definite peak of protein and in three sugar peaks. Fractionation of the protein peak on DEAE-cellulose produced four more protein peaks and another five sugar peaks. Twenty-one untreated allergic patients suffering from perennial rhinitis and bronchial asthma with clinical sensitivity to house dust were subjected to skin tests with house dust 1:100, dermatophagoides farinae 1:100 and also its thirteen eluted fractions 1:100. Positive results were recorded with house-dust, mite and six or perhaps seven fractions consisting of three or four proteins and three sugars. Immunization in rabbits with mite extract revealed only one slight precipitin band by the Ouchterlony technique and a 1:20 titer of hemagglutinating antibodies. The antigen or antigens of mite extract seemed to be producers of type I reactions rather than circulating antibodies of the IgG class. This represented a point of controversy for the therapeutic application of Dermatophagoides farinae in human desensitization.