SEVERAL workers have reported the isolation of a red coloured protein (‘red protein’) from bovine1–3 and human4–8 milk. The occurrence of a similar red protein in serum, responsible for transport of iron (transferrin), is now well established9. It has been suggested10 but not always accepted11 that serum transferrin occurs to some extent also in milk. In the work reported here an attempt has been made to isolate the different red proteins occurring in bovine milk and to compare them with one another and with bovine serum transferrin.