Using high-speed gel filtration chromatography, an aqueous extract of human dander was divided into four fractions (fractions I to IV). Scratch tests with the crude human dander extract and each fraction were performed in 147 patients with atopic dermatitis and 100 control subjects. The crude extract provoked positive reactions in 61.9% (91/147) of patients with atopic dermatitis and 6% (6/100) of control subjects. The most potent allergenic component was present in fraction II, which induced positive reactions in 61.2% (90/147) of patients with atopic dermatitis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis demonstrated that fraction II contained two proteins with molecular weights of 10,000 and 13,000, respectively.