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The soybean allergenic protein, Gly m Bd 30K [Ogawa et al., J. Nutr. Sci. Vitamihol., 37, 555-565(1991)] which is most strongly and frequently recognized by the IgE antibodies in sera of soybean-sensitive patients with atopic dermatitis, has been characterized. The allergen was isolated from the crude 7S-globulin fraction as an oligomeric form with a molecular weight of more than 3000,000 by gel-filtration chromatography. On two-dimensional gel electrophoresis, the native oligomeric allergen had an isoelectric point of about pH 4.5 and was dissociated into a monomeric form with a molecular weight of about 32,000 by the treatment with sodium dodecyl sulfate and 2-mercaptoethanol. The monomeric allergen had an N-terminal amino acid sequence and amino acid composition identical with those of the soybean seed 34-kDa oil-body-associated protein or the soybean vacuolar protein P34 with close homology to papain-like thiol proteinases [Kalinski et al., J. Biol. Chem., 267, 12068 (1992)]. The identity was further confirmed by the immunological cross-reactivity to the antibodies produced against each of the purified allargen and the 34-kDa oil-body-associated protein. By this observation, Gly m Bd 30K was shown to have about 30% sequence homology with Der pI, a house dust mite allergen that is a thiol proteinase from Dermatophagoides pteronyssius.