After methanol extract (CUP-2) with intestinal immune system-modulating activity through Peyer's patch was prepared from Citrus unshiu peel, CUP-2C-2–2 was purified by solvent partitioning, silica gel 60 and Sephadex LH-20 chromatographies (Pharmacia Biotech, Uppsala, Sweden). Ultraviolet-visible spectral, 1H/13C-NMR, and fast atom bombardment mass spectrometry analyses suggested that CUP-2C-2–2 was identified to flavonoid glycoside (5,7,3′,4′-tetrahydroxyflavone linked by glucose) with molecular weight of 725 Da. When the active chloroform layer (CUP-2C) fractionated from CUP-2 was orally administered in mice, the potent activity was observed on the fifth day of 2.0 g/kg body weight/day (1.59-fold of the saline control). Granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-6 contents in the culture medium of Peyer's patch cell were also significantly increased (1.80- and 2.20-fold, respectively) compared with the saline control. These results suggested that flavonoid glycoside from C. unshiu peel might stimulate Peyer's patch and produce the multifunctional hematopoietic growth factors such as GM-CSF and IL-6 followed by proliferation of bone marrow cells.