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We have already reported the effect of ultraviolet-A (UVA)-sensitization of hematoporphyrin (HP) on the production of lipid hydroperoxides in erythrocytes and on their hemolysis. In this report, we investigated these effects under ultraviolet-B (UVB) irradiation and compared the results with those under UVA irradiation. It was found that an increase in lipid hydroperoxide preceded hemolysis, and that the UVB irradiation resulted in an increase in phosphatidylethanolamine hydroperoxide (PEOOH) and 2-thiobarbituric acid reactive substances (TBA-RS) at an earlier stage in comparison with UVA irradiation. Under either UVA or UVB irradiation, hemolysis was inhibited by anti-oxidants such as sodium azide and ascorbic acid (singlet oxygen scavengers), but not by mannitol, sorbitol (hydroxyl radical scavengers) or superoxide disumutase (SOD) (superoxide radical scavenger). These results suggest that singlet oxygen (^1O_2) produced by UV irradiation peroxidized the lipids of erythrocyte membranes, and therefore, the hemolysis of erythrocytes occured when the amount of hydroperoxides increased to a constant level, but auto-oxidation of lipids did not affect the hemolysis.