This paper is the fourth in a series of reports on the Fukuryu Maru No. 5 Incident. Two of the previous reports dealt only with medical aspects; while the third report (by Mr. Merril Eisenbud) dealt with the monitoring of ships, cargoes and crews, and also more with the radiation chemistry and nuclear physics as they pertained to the incident. This fourth report deals with the human element as evidenced by the poor rapport between the American and Japanese teams. This in no small part was due to the rantings of the hysterical sensation-seeking, irresponsible, sometimes mendacious Japanese press. These four reports must be read as a unit; to take one separately would give a false impression of what transpired. The American team was truthful, trustful, and terribly naive. The leaders of this team represented years of training and experience in the finest medical schools and research institutions of the United States. It was inherent in their training that matters of science had to be dealt with openly and honestly. They were obviously not skilled in chicanery and distortion. The deterioration of relationships between the Americans and the Japanese can best be followed against the background of newspaper reports, public utterances of Japanese investigators, resumé of bi-national confer- ences, and selections from the American team's diaries. However, this portion of the entire episode was played against a backdrop of much more political tinge; such factors as the purging of Japanese scientists by SCAP, the stagnation of Japanese medicine, the founding of the Japanese 0eT toe Fad bw bbweee re NIH (National Institute of Health), ABCC in Japan, the return to scientific