Mammalian cell culture in a serum free medium requires an iron transporter such as transferrin. Human transferrin is a 77 kDa beta-glycoprotein with a range of phenotypes based upon the number of sialic acid residues. Each molecule of transferrin specifically binds two Fe3+ molecules. The iron content can be adjusted to give near 100% saturation to yield holo-transferrin (Iron 1200-1600 ug/gm) or depleted to give near zero iron bound to yield apo-transferrin (Iron <50ug/gm protein) or the intermediate- sidero-transferrin (Iron 300-1000ug/gm protein). The secondary role for transferrin in culture media is to bind endogenous metal iron which may cause cell toxicity.