On January 9, 1945, at the University of Rochester Hospital in New York, medical records show that Dr. Joseph G. Hamilton administered an intravenous injection of radioactive phosphorus (P-32) to a hospitalized patient. This event is widely cited by historians and policy analysts as the first documented intentional human radiation experiment in the United States. It occurred in the context of wartime research interest in the biological effects of ionizing radiation and in the early development of medical uses for radioisotopes. Background During the 1930s and 1940s, physicists and physicians began to explore medical applications for newly available radioisotopes, including phosphorus-32, iodine-131 and others. These isotopes had potential for diagnosing and treating diseases because they concentrated in particular tissues. At the same time, the biological hazards of ionizing radiation were not yet fully defined, and oversight mechanisms that are familiar today did not exist. The January 1945 case Contemporary documentation and later historical accounts identify the January 9, 1945 injection as an intentionally administered radiaoactive tracer given to a patient for clinical or investigational purposes. The procedure involved P-32, a beta-emitting isotope used at the time to study metabolic and hematologic conditions because it localizes to bone marrow and other phosphorus-rich tissues. The specific clinical indications and full consent records for that individual case are not comprehensive in surviving sources; subsequent scholarship emphasizes that consent practices and record-keeping of the era were inconsistent by modern standards. Significance and aftermath The January 1945 injection has been cited in historical reviews and government inquiries as a landmark date because it is among the earliest clearly documented instances in which a human subject was deliberately given a radioactive substance in a U.S. medical setting. The case presaged a rapid expansion of human studies involving radioisotopes in hospitals and research institutions, often with limited formal oversight. In the decades that followed, ethical concerns about consent, risk disclosure and long-term follow-up prompted changes: institutional review boards, informed consent requirements and federal regulations governing human subjects research were developed in response to many kinds of ethical lapses, including radiation studies. Uncertainties and historiography While January 9, 1945 is commonly referenced in secondary literature as the first documented human radiation experiment in the U.S., historians note limitations in the archival record. Earlier uses of radiation in clinical care (for example, diagnostic X-rays and therapeutic radium applications) were well established by then, and some experimental exposures occurred in less well-documented settings. Therefore, scholars qualify the claim: January 9 represents the earliest clearly documented intentional injection of a radioactive isotope into a hospitalized patient for investigational purposes in existing records, rather than an absolute statement that no prior experiment ever took place. Why it matters today This early case is used in histories of medical ethics and radiation policy to illustrate how quickly powerful technologies can outpace ethical and regulatory frameworks. It underscores the need for transparency, informed consent and long-term monitoring when medical interventions carry unknown risks — principles that now govern human-subjects research but were developed in part because of controversies that followed mid-20th-century practices. Sources and further reading This summary relies on published historical reviews, institutional records cited in scholarly literature, and analyses produced during later governmental reviews of human radiation experiments. For detailed archival citations and scholarly discussion, consult peer-reviewed histories of medical use of radioisotopes and government reports on human radiation experiments compiled in the late 20th century.