John James Rickard Macleod, FRS (6 September 1876 – 16 March 1935) was a Scottish biochemist and physiologist. He devoted his career to diverse topics in physiology and biochemistry, but was chiefly interested in carbohydrate metabolism. He is noted for his role in the discovery and isolation of insulin during his tenure as a lecturer at the University of Toronto, for which he and Frederick Banting received the 1923 Nobel prize in Physiology or Medicine. Awarding the prize to Macleod was controversial at the time, because according to Banting's version of events, Macleod's role in the discovery was negligible. It was not until decades after the events that an independent review acknowledged a far greater role than was attributed to him at first.
John Macleod was a distinguished physiologist even before the discovery of insulin. He was elected a member of the Royal Society of Canada in 1919 and president of American Physiological Society in 1921. Among the recognitions he received after 1923 were memberships of the Royal Society and the Royal Society of Edinburgh, corresponding membership of the German Academy of Sciences Leopoldina and honorary membership of the Regia Accademia Medica. His reputation in Canada remained under the influence of Banting's story for decades, so Macleod was not esteemed there. His contribution to science is now recognized by the broad public, even in Canada. The auditorium of the Toronto University Medical Research Center was named in his honor, as was Diabetes UK's award for patients who survive for 70 years with diabetes. In 2012, he was inducted into the Canadian Medical Hall of Fame.