This study of strontium-90 retention in infants was undertaken by the Research Branch of the Division of Radiological Health (now the Bureau of Radiological Health) to contribute data toward the development of radiation protection standards for what may be the most radiosensitive age sector of the population. The study was begun in 1960, at a time when national and international agencies– the Federal Radiation Council, National Committee on Radiation Protection, and International Commission on Radiological Protection–were particularly interested in obtaining metabolic information for predicting internal exposure from the ingestion of strontium-90 deposited in world-wide fallout. The long-term metabolic balance study was feasible because the strontium-90 content in the normally available food was sufficiently high to be determined accurately when measured with care. The advantages of determining retention under actual home conditions without altering the environment and the diet of the infants were believed to outweigh the known difficulties and the inherent errors in such a balance study. The 4-year experimental program was performed by Radiological Health Research Activities, Cincinnati, Ohio. It enjoyed the full support of the Chiefs of the Research Branch, Dr. Paul Tompkins and Dr. Arthur Wolff. Specific participation is noted in the Acknowledgment; in addition, many other staff members of Radiological Health Research Activities contributed to the study. The authors cooperated in all phases of the work, but some specialization may be recognized: Dr. Conrad P. Straub conceived, planned, and directed the entire study; Drs. Norman C. Telles, Henry N. Wellman, and Ronald A. Seltzer were the medical officers directly responsible for the study–initiated and supervised by Dr. Telles during the first year, by Dr. Wellman during the second and third years, and by Dr. Seltzer during the final year; Mr. Phillip J. Robbins directed the analytical program at the end of the study; and Dr. Bernd Kahn was responsible for providing analytical procedures and writing the report.