The effects of alpha-tocopherol and related compounds on the membrane fluidity of liposomes composed of dipalmitoyl-lecithin or egg lecithin were studied by measuring the fluorescence polarization of DPH(1,6-diphenyl-1,3,5-hexatriene), a hydrophobic fluorescent probe distributed in the hydrocarbon region of lipid bilayers. The transition temperature of dipalmitoyl-lecithin liposomes was lowered on incorporation of alpha-tocopherol into the membranes. A similar effect was observed on incorporation of phytol or phytanic acid into liposomes, but not on incorporation of 2,2,5,7,8-pentamethyl-1-hydroxychroman as an alpha-tocopherol model compound (TMC) which lacks an isoprenoid side chain. Above the phase-transition temperatures, incorporation of alpha-tocopherol or TMC dose-dependently increased the polarization values of DPH in dipalmitoyl-lecithin and egg lecithin liposomes, while incorporation of phytol or phytanic acid had little effect or slightly lowered the polarization. These results suggest that the effect of alpha-tocopherol in lowering the phase-transition temperature depends on its hydrophobic side chain, and that its effect in decreasing the membrane fluidity above the phase-transition temperature depends on its hydrophylic chroman ring portion. Cholesterol had an effect similar to alpha-tocopherol on membrane fluidity. alpha-Tocopherol and cholesterol had independent effects in increasing the polarization of DPH above the phase-transition temperature when the two compounds were incorporated together into liposomes.