William's Medium E contains unique ingredients such as zinc, iron, manganese, non-essential amino acids, the reducing agent glutathione and the lipid methyl linoleate. William's E Medium can be supplemented, usually with 5-10% fetal bovine serum may be required. William's E Medium uses a sodium bicarbonate (2.2 g/l) buffer system and therefore requires a 5-10% CO 2 environment to ma physiological pH.