Allergy-preventive activity of flower buds of Lonicera japonica THUNB. was found in the 35% EtOH extract (LJ) using an in vivo assay, The assay system uses monitoring of a decrease in blood flow (BF) in the tail vein of mice subjected to sensitization with hen-egg white lysozyme (HEL). Bioassay-guided fractionation of the 35% EtOH extract led to isolation of chlorogenic acid (1) and three known iridoid derivatives, loganin (2), secoxyloganin (3) and sweroside (4), all of which inhibited the BF decrease. This suggested that the flower buds of L. japonica and compounds isolated from them have allergy-preventive properties. The structure–activity relationship of iridoid derivatives, morroniside (5), geniposide (6), asperuloside (7), aucubin (8) and catalpol (9), were also tested using the same bioassay method. Compounds 2—5 and 9 having the sp³ atom at C-8 showed an allergy-preventive effect, while compounds 6, 7 and 8 having a double bond at C-7, C-8 did not.