Fruits of Sophora japonica L. (Leguminosae) exhibited an inhibitory effect in the IL-5 bioassay of mIL-5-dependent Y16 proliferation. The isoflavonoids of sophoricoside, genistein, orobol, and genistin were isolated as the IL-5 inhibitors from fresh fruits of the plant by activity-guided fractionation. Among the IL-5 inhibitors, sophoricoside exhibited the highest inhibitory effect with 89 % inhibition at 12.5 μM, 82 % at 6.3 μM, 72 % at 3.1 μM, 59 % at 1.6 μM, and 24 % at 0.8 μM where the 50 % inhibition (IC50) was shown at the concentration of 1.5 μM. Oxyphenylbutazone as the positive control exhibited the IC50 value at the concentration of 31.7 μM. In the order of IC50 values the inhibitory potency in the IL-5 bioassay was sophoricoside > orobol (9.8 μM) % genistin (10.6 μM) > genistein (51.9 μM). The position of O-glycosylation and number of hydroxy groups in the isoflavonoids seem to play an important role in the inhibitory effect in the IL-5 bioassay.