THE presence of cæsium-137 in human beings was established in the United States in 1955 (ref. 1) and measurements of the average levels in a small group of unexposed subjects have been made at this Establishment since early 1956. The results of these measurements showed that there was a slow, irregular increase in the cæsium-137 content to a maximum in the summer of 1959, after which the level declined to a lower value in April 1962 than at any time since our measurements began2,3. The resumption in September 1961 of largescale atmospheric testing of high-yield nuclear weapons did not result in any detectable increase in cæsium-137 contents until after April 1962, although the presence of short-lived zirconium-95 in human beings was demonstrated prior to this3. In this communication we are reporting the results of our measurements of cæsium-137 in human beings during April 1962-April 1964; we have used throughout the technique described previously3, which was intended primarily for estimating the lung contents of zirconium-95/niobium-95. The results of serial measurements of the latter radionuclides will be reported elsewhere.