Project 4.1 was the designation for a medical study and experimentation conducted by the United States of those residents of the Marshall Islands exposed to radioactive fallout from the 1 March 1954 Castle Bravo nuclear test at Bikini Atoll, which had an unexpectedly large yield. Government and mainstream historical sources point to the study being organized on March 6 or 7 March 1954, less than a week after the Bravo shot.