Distribution of ^<35>S in plasma and red cells was investigated after TPD-^<35>S (outer or inner) or thiamine-^<35>S was administrated to rabbits. After oral or parenteral administration of TPD-^<35>S (outer), ^<35>S was found mainly in plasma, and thiamine was mostly in red cell. ^<35>S in plasma was found to be combined with serum albumin. ^<35>S increased remarkably in the red cell after an administration of TPD-^<35>S (inner).