Robert Kenneth Callow, FRS (15 February 1901 – 1983) was a British biochemist. He worked at the National Institute for Medical Research (NIMR, Medical Research Council) in Hampstead and Mill Hill, where his work on steroids included contributions to the isolation and characterisation of vitamin D, and the synthesis of cortisone from naturally occurring steroids. After he retired from the NIMR in 1966 he worked on insect pheromones at Rothamsted Experimental Station (now Rothamsted Research) until 1971. Early life and education Kenneth Callow was born 15 February 1901 in Goring-on-Thames, Oxfordshire, England. His father, Cecil Callow (1865–1912), was an electrical engineer. Kenneth's mother, Kate Peverell (1868–1955), became the head of the Peverell household in Gateshead in 1885 after her parents died, when she was 17 years old with two younger sisters. After 1891 she moved to London. In 1896 she married Cecil Callow. Kenneth attended City of London School (1911–1919), on a scholarship after his father died in 1912. He was awarded a senior science scholarship in 1916, that allowed him to continue at school. In 1919 he went up to Christ Church, Oxford as an exhibitioner, to study chemistry. For part II chemistry his supervisor was Nevil Sidgwick, FRS. After a time at British Celanese (1924–1927) he returned to Oxford as a Research Scholar of Christ Church, synthesising alkaloids and attempting to isolate taxine, a toxic alkaloid of yew. He submitted his D.Phil. thesis in 1929. Vitamin D In 1929 he was invited by R.B. Bourdillon at NIMR in Hampstead to join him in work on vitamin D. The structure of vitamin D was unknown at that time, and the structure of steroids in general was a matter of debate. A meeting took place with J.B.S. Haldane, J.D. Bernal and Dorothy Crowfoot to discuss possible structures, which contributed to bringing a team together. X-ray crystallography demonstrated that sterol molecules were flat, not as previously proposed by Adolf Windaus. In 1932 Otto Rosenheim and Harold King published a paper putting forward structures for sterols and bile acids which found immediate acceptance. The loose association between Bourdillon, Rosenheim, King and Callow was very productive and led to the isolation and characterisation of vitamin D. At this time the policy of the MRC was not to patent discoveries, believing that results of medical research should be open to everybody. The team working on vitamin D included Bourdillon, a physical chemist with a