The year 1940 in science and technology involved some significant events, listed below.
August 24 – Howard Florey and a team including Ernst Chain, Arthur Duncan Gardner, Norman Heatley, M. Jennings, J. Orr-Ewing and G. Sanders at the Sir William Dunn School of Pathology, University of Oxford, publish their laboratory results showing the in vivo bactericidal action of penicillin. They have also purified the drug. On December 25 they seed their first batch of culture with spores of penicillin to grow it in medicinal quantity.
The antibiotic dactinomycin (actinomycin D) is first isolated by Selman Waksman and H. Boyd Woodruff at Rutgers University.
February 2 – The first transposons are discovered in maize (Zea mays, aka corn) by Barbara McClintock.
March 11 – Ed Ricketts, John Steinbeck and six others leave Monterey for the Gulf of California on a marine invertebrate collecting expedition.
February 27 – The radioactive isotope carbon-14 is discovered by Martin Kamen and Sam Ruben at the University of California, Berkeley.
May 15 – Women's stockings made of nylon are first placed on sale across the United States.
December 14 – Plutonium is first synthesized by a team led by Glenn T. Seaborg and Edwin McMillan at the University of California, Berkeley by bombarding uranium-238 with deuterons.
The radioactive element Astatine is synthesized by Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè at the University of California, Berkeley.
Neptunium, the first transuranic element, is synthesized by Edwin McMillan and Philip H. Abelson at the University of California, Berkeley.
Louis Plack Hammett coins the term Physical organic chemistry when he uses it as the title of a textbook published in New York.
Robert McCance and Elsie Widdowson publish the standard text The Chemical Composition of Foods.
January 8 – In the history of computing hardware, Bell Labs' Complex Number Computer, a relay-based calculator for complex numbers, is completed under the direction of George Stibitz in New York City.