As a Department of Energy Committee writing on the human radiation experiments wrote, "It appears to have been almost immediately apparent to the AEC and the Joint Task Force running the Castle series that research on radiation effects could be done in conjunction with the medical treatment of the exposed populations." The DOE report also concluded that "The dual purpose of what is now a DOE medical program has led to a view by the Marshallese that they were being used as guinea pigs in a radiation experiment."
Organizations involved in the project included the Naval Medical Research Institute, the Naval Radiological Defense Laboratory, Patrol Squadron 29, the Naval Air Station, Kwajalein, Los Alamos National Laboratory, the Applied Fisheries Laboratory at the University of Washington, and Hanford Atomic Power Operations. Three U.S. Navy ships were used in the project: USS Nicholas, USS Renshaw, and USS Philip. The primary study of the Marshallese was terminated around 75 days after the time of exposure. In July 1954 a meeting at the Division of Biology at the U.S. Atomic Energy Commission decided to complete 6- and 12-month follow-up exposure studies, some of which were later written up as addendums to Project 4.1.