Prof. MUDr. Jan Janský (Czech pronunciation: [ˈjan ˈjanskiː]) (3 April 1873 in Prague – 8 September 1921 in Černošice, near Prague) was a Czech serologist, neurologist and psychiatrist. He is credited with the first classification of blood into the four types (A, B, AB, O) of the AB0 blood group system.
Janský studied medicine at Charles University in Prague. From 1899, he worked in the Psychiatric Clinic in Prague. In 1914, he was named professor. During World War I Janský served two years as a doctor at the front until a heart attack disabled him. After the war he worked as a neuropsychiatrist in a military Hospital (Vojenská nemocnice). He had angina pectoralis and died of ischaemic heart disease.
Janský was also a proponent of voluntary blood donations.
Through his psychiatric research, Janský tried to find a correlation between mental diseases and blood diseases. He found no such correlation existed and published a study, Hematologická studie u psychotiků (1907, Hematological study of psychotics), in which he classified blood into four groups I, II, III, IV. At the time this discovery passed almost unnoticed. In 1921 an American medical commission acknowledged Janský's classification (over that of Karl Landsteiner, who classified blood into only three groups; and was for this (blood types) discovery awarded the Nobel Prize in Physiology or Medicine in 1930). Janský's classification remains in use today. A similar classification was described by William Lorenzo Moss, except the I and IV of Moss were the opposite to that of Janský's, leading to confusion in blood transfusion until the use of A, B and O became standard.