Born on 8 August 1881 at Lockfield near Darvel, Ayrshire, the son of a farmer, he was educated first at the village school and later at Kilmarnock Academy. At the age of 13 he was sent to live with his brother in London and continued his education at the Polytechnic Institute in Upper Regent Street where he displayed no particular interest in science or desire to become a doctor. Following this he worked for four years in a shipping office in Leadenhall Street, but then a small legacy enabled him to escape from the dull routine and, following the lead of his brother who had by now taken a medical degree, he entered the medical school of St Mary's, gaining a senior entrance scholarship in natural science. During his student career he won nearly every prize and scholarship, and finally was awarded a gold medal in the London MB examination. After qualification he began working in Sir Almroth Wright's inoculation department and, having achieved his FRCS in 1909, he decided to take up bacteriology under Wright's stimulating direction. Two more dissimilar men it would be hard to imagine: Wright the brilliant, dialectic Irishman, Fleming the dour, cautious lowland Scot. While employed by the shipping company, he had joined the London Scottish as a private and regularly attended their annual meetings, including, as a competent shot, the meetings at Bisley. In August 1914 he transferred to the RAMC, going to France as a Captain to work in Wright's laboratory situated in the Casino at Boulogne, for which service he was mentioned in dispatches. After the war he returned to St Mary's and was appointed lecturer in bacteriology. Later he became director of systematic bacteriology and assistant director of the inoculation department. In 1928 he was appointed Professor of Bacteriology at St Mary's, retiring as emeritus professor in 1948 but continuing as head of the Wright-Fleming Institute of Microbiology. This appointment he relinquished in 1954, but continued to work in the laboratory up to the time of his death. In 1922 he discovered lysozyme and, in 1928, penicillin while engaged in research on staphylococci. He found, however, that crude penicillin was too weak as a therapeutic agent and attempts at concentration were unsuccessful; as a result its clinical use was not pursued. Fleming's original paper (*Brit J exp Path* 1929, 10, 226) nevertheless was remarkable in appreciating most of the problems and their probable solution. It was left to Sir Howard Florey and E B Chain at Oxford to establish penicillin as a therapeutic agent in 1943. Fleming was knighted and shared the Nobel Prize for medicine with Florey and his collaborator Chain. Fleming was elected a Fellow of the Royal Society and a Fellow of the Royal College of Physicians, was awarded the Moxon medal of the College of Physicians, the honorary medal of the College of Surgeons, the Charles Mickle Fellowship of Toronto, the John Scott medal of Philadelphia, the Cameron prize of Edinburgh University, the Albert gold medal of the Royal Society of Arts, and the Actonian prize of the Royal Institution. Many honorary degrees of British and foreign universities were conferred upon him, and in 1951 he was elected Rector of Edinburgh University. In 1946 he acted as President of the Inter-American Medical Congress in Rio de Janeiro, and was awarded the Brazilian Order of the Southern Cross. He was President of the section of pathology of the Royal Society of Medicine and of the London Ayrshire society. He remained modest, simple and unspoilt, when success finally came to him after years of methodical work, and he gave full credit to the part played by other investigators. In his leisure he was a painter, gardener, motorist and Freemason and a member of the Chelsea Arts Club. In his younger days he was a crack shot and a first rate swimmer. He married first Sarah Marion, daughter of John McElroy of Killala, Co Mayo, who died on 28 October 1949, by whom he had a son, a doctor; and secondly in 1953 Dr Amelia Coutsouris of Athens, who had worked in his department. He died on 11 March 1955 at his home in Chelsea, and his ashes were placed in the crypt of St Paul's. A memorial service was held in St Paul's Cathedral on 18 March and at St James's, Sussex Gardens on 24 March 1955.