The chemical constituents of Japanese mistletoes, Taxillus yadoriki Danser, Taxillus kaempferi Danser, and Korthalsella japonica Engler, epiphyting to different host trees were compared, and the antimicrobial and hypotensive properties of some isolated flavonoids were examined. Two known flavonoid glycosides, hyperin and quercitrin, were isolated from Taxillus yadoriki Danser, together with fatty acids, phytosterol, and phytosterol-glucoside. There was remarkable variation of contents of quercitrin among the plants on different host trees. From Taxillus kaempferi Danser, fatty acids, phytosterol, phytosterol-glucoside, quercetin, avicularin, and taxillusin were isolated, and quercitrin and hyperin were also identified. There was no remarkable variation of compositions of flavonoid glycosides among the plants on different host trees. A known flavone glycoside, chrysoeriol-4'-O-glucoside, was isolated from Korthalsella japonica Engler, together with fatty acids, phytosterol, oleanolic acid, and phytosterol-glucoside. Chrysoeriol-4'-O-glucoside is contained in this plant irrespective of the host trees.