By the end of the Second World War the advancing allied forces discovered a new nerve gas in Germany. It was called Tabun. Codenamed GA, it was found to be extremely toxic. British experts were immediately dispatched to examine the agent. On arrival, they discovered that German scientists had also developed even more toxic nerve agents, including Sarin, known as GB. The first organized testing of Sarin on humans began in October 1951 at Porton Down in Wiltshire, Britain's biochemical warfare establishment since the First World War. In February 1953, volunteer number 562 experienced the first recorded serious adverse reaction. Testing continued. Two months later, on April 27, six subjects were given 300 milligrams of Sarin. One of the volunteers, a man named Kelly, suffered serious ill effects, fell into a coma, but then recovered. Although asked by their superiors to reduce the amount tested to the “lowest range of dosage”—which would have been somewhere in the region of 10–15 milligrams—Porton's scientists continued their tests with a “lower” dosage, reducing it from 300 to 200 milligrams.I thank the two referees for their helpful and constructive suggestions. The paper was written as part of a Wellcome Trust-funded project on “Cold War at Porton Down: Medical Ethics and the Legal Dimension of Britain's Biological and Chemical Warfare Programme, 1945–1989.” I thank the Wellcome Trust for its generous support.