Allergy-preventive activity was demonstrated for the MeOH extract (HM) of the petals of Hibiscus mutabilis L. ‘versicolor’ MAKINO in a continuing search for allergy-preventive substances from natural sources, using the in vivo assay method. This assay system uses monitoring of a decrease in the blood flow at the tail vein of mice subjected to hen egg-white lysozyme (HEL) sensitization. By bioassay-directed fractionation, a new flavonol triglycoside, quercetin 3-O-[β-D-xylopyranosyl(1→2)-α-L-rhamnopyranosyl(1→6)]-β-D-galactopyranoside (1: mutabiloside), was isolated, together with four known flavonols identified as quercetin 3-O-[β-D-xylopyranosyl(1→2)]-β-D-galactopyranoside (2) and kaempferol 3-O-[β-D-xylopyranosyl(1→2)]-β-D-galactopyranoside (3), quercetin (4) and hyperoside (5). The structure of the new flavonol 1 was elucidated by spectroscopic methods. Among these flavonol derivatives, compounds 1 and 2 showed significant allergy-preventive effects.