text: 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 March 1, 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 March 7, 1954, less than a week after the Bravo shot.