William Castle, known to all as the discoverer of the cause of pernicious anaemia, was born in Cambridge, Massachusetts, on 21 October 1897. His father, William Ernest Castle, was a professor of zoology at Harvard and a pioneer in mammalian genetics. His mother was Clara Seers Bosworth, the daughter of a farmer.

He became a medical intern at Massachusetts General Hospital, an instructor in physiology at Harvard medical school, assistant resident in the Thorndyke Memorial Laboratory (1925 to 1926), Boston City Hospital, as well as being assistant in medicine at Harvard medical school. He then became associate professor of medicine at Harvard (1932 to 1937), professor of medicine at Harvard (1937 to 1957), and director of the Thorndyke Memorial Laboratory from 1948 to 1963. He was also director of medical services at Boston City Hospital, George R Minot Professor of Medicine from 1957 to 1963, professor of medicine in the Francis Peabody Faculty, and finally honorary director of the Thorndyke Memorial Laboratory from 1963 onwards.

It was from the Thorndyke Memorial Laboratory that he published his seminal work, 'The Etiology of pernicious anaemia and related macrocytic anaemias', Annals of Internal Medicine, 1935, 7: 1, 2-5. Following this, he and his colleagues George Minot and William Murphy won the Nobel Prize for Medicine in 1934, for the discovery of the cure of pernicious anaemia, raw liver or liver extract. Many honours followed including the Warren Triennial Prize of Massachusets General Hospital in 1928, the Philips Memorial Prize of the American College of Physicians in 1932, the Walter Reed Medal of the American Society of Tropical Medicine in 1939, and many others. He was special adviser to the committee on medical research of the Office of Scientific Research and Development during World War II.

In 1933, he married Louise Griffen Mailer whose father was a merchant-tailor. They had two children, William R, and Anne Morris. In biographical data he submitted on becoming an honorary fellow in 1964, he mentioned his hobbies as water colours and sailing. It is of interest that so eminent a man filled out his own admission sheet for the College in his own handwriting. It is fitting to end this biography with a quote from Dr David Jean Nathan, physician in chief at Children's Hospital, Boston, and former student of Dr Castle's:

'He did everything himself...including repairing the plumbing in the BCH Thorndyke Lab which was always in need of repairs...a brash young fellow who came into the Thorndyke to start his training went over to a sink and saw a man repairing the drain, "Could you hurry that up? I'd like to get started", the brash young man said. "Oh yes sir, I'll get right along", came the reply. It was Dr Castle.'