(Robert) Gwyn Macfarlane (1907-1987) was a clinical pathologist and haematologist who trained at St Bartholomew's medical school in London and who built his career and reputation through his research and expertise on blood coagulation, and particularly on the treatment of haemophilia.

In 1940 Macfarlane became the clinical pathologist at the Radcliffe Infirmary, Oxford, where he remained until his retirement in 1968. During the Second World War, he helped to develope national standards and procedures for the measurement of haemoglobin levels in blood and organised the local blood transfusion service, where he worked under Dr. Ethel Florey (the wife of a man who became one of Macfarlane's friends, penicillin nobel laureate Howard Florey) and alongside Sir William Dunn School of Pathology staff including the third penicillin nobel laureate, Ernst Chain.

In 1956, Macfarlane was elected fellow of the Royal Society and by 1965 held an Oxford chair in clinical pathology. He retired to Ross-shire in 1977 but continued to champion the treatment of haemophilia. He also completed two medical biographies, Howard Florey: the Making of a Great Scientist (1979) and Alexander Fleming, the Man and the Myth (1984), which was a major re-evaluation of Fleming's work on penicillin and of his scientific standing.Through these became involved in documentaries and science journalism, including the 1986 BBC Horizon documentary The Mould, the Myth and the Microbe, about the development of penicillin.

[Adapted from the Oxford DNB article on Macfarlane by Michael Worboys.