A rapid transport and the accumulation of O-acetoglycoloyl-S-furoylthiamine (AFT) in human red cells were found and the detailed mechanism was investigated. AFT-^<35>S and thin layer chromatography were employed to clarify the metabolism of AFT in and outside the red cells and a rapid decomposition of AFT to undiffusible thiamine in the cells was found to be responsible for the accumulation phenomenon. S-Furoyl group was found to be decomposed mainly in non-enzymatic way, by intracellular glutathione, while O-acetoglycoloyl group by intracellular, and to a lesser extent by membrane, esterase, only the latter being strongly inhibited by di-isopropylfluorophosphate.