The house dust mite, Dermatophagoides farinae, was fractionated by a Sephadex G-200 column. Its allergenic (IgE-reacting) and immunogenic (IgG-reacting) components were investigated. By means of skin test, the molecular weight (MW) of major allergenic components of mite was found to be approximately 9,000 to 21,000 daltons. Immunogenic components were investigated by enzyme linked immunosorbent assay using each fraction as an antigen and mice plasma and human serum as antibodies. With mouse plasma, high IgG antibody titers were observed in fractions that contained the part of the mite with high MW (greater than 150,000). With human sera, high IgG antibody titers were observed in fractions that contained the part of the mite with MW more than 30,000. Heterogeneity of human IgG antibody responses against mite antigen was also suggested.