β-Carotene has been known as an effective quenching agent of singlet oxygen and the carotenoid pigments in general are expected to protect cells against photosensitized oxidations. We are determined the quenching rate constants of several Ketocarotenoids including capsanthin, capsanthin diester. astaxanthin and fucoxanthin, and the relative quenching activities against singlet oxygen were compared with those of β-carotene and reported carotenoids. Nevertheless the ketocarotenoids exhibited lower quenching rate constants than β-carotene, they showed more pronounced protective activitives than β-carotene against photohemlysis induced by singlet oxygen. Among the ketocarotenoids investigated, fucoxanthin indicated a significant protective activity for the cell. The results suggested that. 1) ¹O₂ may be alikely initiator of photohemolysis, but this reaction is followed by slow dark reactions involving secondary reactive species. 2) For protection of RBC against photodynamic action with carotenoids, carotenoids having functional groups such as -C=O and -OH groups are most efficient. This suggests that partition of carotenoids between the bulk and the membrane and/or their specific binding to membrane proteins are more critical for the photo-protection by carotenoids than is a diffusional quenching of ¹O₂.