NOBEL PRIZE WINNERS: A good multiple signed First Day of Issue cover honouring the Mayo Brothers and featuring their images and with a postage stamp designed in their honour, post marked at Rochester, Minnesota, 11^th September 1964, individually signed by eleven Nobel Prize winners in Physiology or Medicine comprising Edgar Adrian (1889-1977, 1^st Baron Adrian, English electrophysiologist, Nobel Prize winner for Physiology or Medicine, 1932), William P. Murphy (1892-1987, American physician, Nobel Prize winner for Physiology or Medicine, 1934), Albert Szent-Gyorgyi (1893-1986, Hungarian biochemist, Nobel Prize winner for Physiology or Medicine, 1937), Henrik Dam (1895-1976, Danish biochemist and physiologist, Nobel Prize winner for Physiology or Medicine, 1943), Edward A. Doisy (1893-1986, American biochemist, Nobel Prize winner for Physiology or Medicine, 1943), Ernst Chain (1906-1979, German-British biochemist, Nobel Prize winner for Physiology or Medicine, 1945), Carl F. Cori (1896-1984, Czech-American biochemist and pharmacologist, Nobel Prize winner for Physiology or Medicine, 1947), Bernardo Houssay (1887-1971, Argentine physiologist, Nobel Prize winner for Physiology or Medicine, 1947), Frederick Chapman Robbins (1916-2003, American paediatrician and virologist, Nobel Prize winner in Physiology or Medicine, 1954), Francis Crick (1916-2004, English molecular biologist, biophysicist and neuroscientist, Nobel Prize winner for Physiology or Medicine, 1962) and Edmond H. Fischer (1920-2021, American-Swiss biochemist, Nobel Prize winner for Physiology or Medicine, 1992). All have signed in various coloured inks to clear areas of the cover, and only one is very slightly affected by the postal cancellation. An impressive grouping of signatures of Nobel Laureates. VG