Daigo Fukuryū Maru (第五福龍丸, F/V Lucky Dragon 5) was a Japanese tuna fishing boat with a crew of 23 men which was contaminated by nuclear fallout from the United States Castle Bravo thermonuclear weapon test at Bikini Atoll on March 1, 1954.

All recovered from the immediate effects of the American test detonation except for Kuboyama Aikichi, the boat's chief radioman, who died on September 23, 1954, from complications of radiation sickness.

On March 1, the map depicts the vessel very near to the border of the US Navy issued "danger zone notice" dated October 10, 1953.

Contemporary references give a figure of "80 miles (130 km) east of Bikini Atoll" without stating the method by which the distance was computed.

However, the test was more than twice as powerful as predicted, and changes in weather patterns blew nuclear fallout, in the form of a fine ash, outside the danger zone.

One fisherman, Oishi Matashichi, reported that he "took a lick" of the dust that fell on his ship, likening the falling material to 粉雪 ("powdered snow") and describing it as gritty but with no taste.

The dust stuck to their bodies and the ship, entering their nasal passages and ears, irritating their eyes and collecting inside their underwear.

One crewman decided to keep some of the ash in order to have it analysed on their arrival home, but it was kept in a pouch hung from one of the bunks and was therefore in close proximity to the sleeping men for the duration of their return.

[13] After their arrival, the men went to the Yaizu Public Hospital, where the surgeon, Oi Toshiaki, applied a zinc ointment to their faces and sent them home.

On March 15, 1954, engineer Yamamoto, deckhand Masuda, and 5 others who were said to make up the "elderly" crew members were sent to the Tokyo University Hospital for treatment.

[14] There, they tested Masuda's bone marrow and found his white blood cell count at half the normal level.

He quickly concluded that they had been exposed to radioactive fallout and wrote a letter to the chief of the US Atomic Energy Commission (AEC) asking for more information on how to treat the crew.

The crew members, suffering from nausea, headaches, burns, pain in the eyes, bleeding from the gums, and other symptoms, were diagnosed with acute radiation syndrome.

However, the United States did dispatch two medical scientists to Japan to study the effects of fallout on the ship's crew and to assist their doctors.

[16] There is a hint of criticism from one of the crewmembers, Oishi Matashichi, aimed at the then Japanese Foreign Minister Katsuo Okazaki in his book, citing the fact that despite the lingering resentment towards the US over the atomic bombings of Hiroshima and Nagasaki in 1945, and the suspicion that US officials were only interested in research rather than attempting to cure anyone of their subsequent bombing-related ailments, Foreign Minister Okazaki is said to have spoken frequently to the crew about the need for the Americans to be present during treatment.

[18] Dr. Morita Hisao reported that the men had developed acute panmyelosis, a disease that attacked their bone marrow destroying its ability to generate blood.

[20] Like the hibakusha, survivors of atomic bombings in Hiroshima and Nagasaki in 1945, the Daigo Fukuryū Maru crew were stigmatized because of the Japanese public's fear of those exposed to radiation (it was commonly believed to be contagious).

For instance, Joseph Rotblat may have deduced the staging nature of the device by studying the ratio and presence of tell-tale isotopes present in the fallout.

He also hypothesized that the lesions on the fishermen's bodies were not caused by radiation but by the chemical action of the caustic burnt lime that is produced when coral is calcined, and that they were inside the danger zone.

He told President Eisenhower's press secretary that the Daigo Fukuryū Maru may have been a "red spy outfit", commanded by a Soviet agent intentionally exposing the ship's crew and catch in order to embarrass the USA and gain intelligence on the test's device.

Despite denials by Lewis Strauss concerning the extent of the claimed contamination of the fish caught by Daigo Fukuryu Maru and other ships, the FDA later imposed rigid restrictions on tuna imports.

[8] The public outcry against the government's handling of the Daigo Fukuryū Maru, its crew, and the lack of information about fallout kindled an anti-nuclear and anti-American movement.

At the first World Conference, a new organization called the Japan Council Against Atomic and Hydrogen Bombs formed to expand the movement and moved to include the hibakusha.

The ship was pulled from the water and put on public display as a symbol of opposition to nuclear weapons in an exhibit hall in Tokyo.