Large unilamellar liposomes comprising of egg yolk phos-phatidylcholine (PC) was exposed to photoirradiation in the presence of methylene blue (water-soluble photosensitizes) or 12-(1-pyrene)dodecan-oic acid (P-12, lipid-soluble photosensitizer) to estimate the inhibitory effect of β-carotene and astaxanthin on photosensitized oxidation of phospholipid bilayers. Without sensitizers, astaxanthin decreased much slower than β-carotene and other hydrocarbon carotenoids (lycopene, a-carotene). Astaxanthin lasted longer than β-carotene even in the presence of methylene blue or P-12. Decrease of astaxanthin was also much slower than that of β-carotene when egg yolk PC was replaced by dimyristoyl PC. However, inhibitory effect of astaxanthin was lower than β-carotene in the case of P-12 sensitized photooxidation. These results suggest that effectiveness of carotenoids as antioxidants on photosensi-tized oxidation (Type II) in phospholipid bilayers depends on the site of singlet oxygen to be generated, as well as their stability on photoirradia-tlon.