===Frederick Banting and the discovery of insulin===At the end of 1920, Macleod was approached by Frederick Grant Banting, a young Canadian physician who had the idea of curing diabetes using an extract from a pancreas whose functioning had been disrupted.
Leta 1903 je odšel v Združene države Amerike, kjer je postal profesor fiziologije na Univerzi Western Reserve v Clevelandu (danes Univerza Case Western Reserve) in tam deloval nadaljnjih 15 let.