Transferrin is the primary iron binding and cell delivery molecule present in serum. Serum-free cell culture systems require a delivery format for iron. Transferrin is the preferential delivery form of iron because cells process transferrin bound iron in a physiologically appropriate way through transferrin receptors on the cell surface. Bovine holo-transferrin can be used with a wide range of cells types. Bovine holo-transferrin is loaded with iron prior to use and can be added directly to iron poor or iron-free cell culture media.