Characterization and purification of the allergenic components of the house dust mite, Dermatophagoides farinae, were investigated. It was found that the mite antigen was heat-stable, pronase-sensitive, and periodate-resistant, which suggested that the antigenic determinants of the mite antigen resided mainly in the protein component. Purification of the mite antigen was tried using Sephadex G-150, DEAE cellulose, and isoelectrofocusing columns. The most potent allergenic fraction was around 20,000–30,000 in molecular weight, although the allergenic activity spread in a wide molecular weight range. Prausnitz-Küstner type skin reactions were performed on rats using mouse homocytotropic antibodies for allergenic evaluation, and the clinical usefulness of this method is discussed.