In 1953, William Seward Burroughs made several important and largely unrecognized discoveries relating to the composition and clinical pharmacological effects of the hallucinogenic plant potion known as yagé or ayahuasca. Illustrations of Burroughs’ voucher sample of Psychotria viridis and his letter to the father of modern ethnobotany, Richard Evans Schultes, are published here for the very first time.