Tetrazolium dye reduction is generally assumed to be dependent on NAD(P)H-dependent oxidoreductase enzymes largely in the cytosolic compartment of the cell.[1][8] Therefore, reduction of MTT and other tetrazolium dyes depends on the cellular metabolic activity due to NAD(P)H flux. Cells with a low metabolism such as thymocytes and splenocytes reduce very little MTT.