Riboflavin, suggested to be a radiosensitizer, was studied in murine thymocytes and human hepatoma L02 cell line in vitro with MTT method and fluorescence microscopy. When the murine thymocytes treated with 5-400 mumol/L riboflavin were irradiated by 5 Gy (60)Co gamma ionizing radiation, the low concentration groups, i.e. treated with 5-50 mumol/L riboflavin, showed a different surviving fractions-time relating correlation compared with the high concentration groups, i.e. treated with 100-400 mumol/L riboflavin. The former had a high survival level at the end of irradiation, but which, after 4-h incubation, decreased rapidly to a low level. On the contrary, the high concentration groups showed a low survival level at the end of irradiation, and a poor correlation was found between the surviving fraction and the incubation time, after 4 h a little difference was observed. The results of fluorescence microscopy indicated that under low concentration conditions, the riboflavin localized mainly in nucleus (both perinuclear area and inside of nuclear membrane), while under high concentration conditions, intensive riboflavin also localized around cytoplasmic membranes. Thus we can conclude: the riboflavin had radiosensitivity effect on DNA under low concentration conditions, and enhanced the damage to cytoplasmic membrane under high concentration conditions. Also the most effective concentration of riboflavin can be evaluated to be approximate 100 mumol/L.