Riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) caused hemolysis of isolated rabbit erythrocytes by UVA-sensitization. Hemolysis induced by UVA-sensitization with RF and FMN was inhibited by 1.0 mM histidine and EDTA, and was promoted by 0.15 mM histidine and 0.01 mM NaN3. Hemolysis induced by irradiation in anaerobic condition was smaller than those in aerobic condition. Hemolysis was proceeded in the dark for 5 hours after the UVA-irradiation with RF and FAD. Erythrocyte treated by UVA-sensitization with RF easily caused hypotonic hemolysis.