Generally, cells do not require mitochondria as demonstrated by the observation that no differences are observed in the purple formazan production by normal cells in which mitochondria have been poisoned by the nucleic acid toxin, ethidium bromide. Since both serum and plasma, from a variety of species, non-specifically reduce MTT tetrazolium salts to a purple colored formazan product. Hence, fetal bovine serum is particularly used in the MTT assay in order to produce purple MTT formazan crystals.