Measurements of radioactivity in five men were made during and after a period of chronic oral exposure (∼O·5 μCi/day) to85Sr. The experimental methods used to determine whole-body contents and skeletal contents are described. The retention up to ∼500 days after the end of the exposure could be well described either by a three-component exponential function of time or by the integrated form of the sum of an exponential function and a power function of time, and the appropriate form of the latter predicted with fair accuracy the build-up of skeletal85Sr during the exposure in the two cases where experimental data were available. The average retention equations were used to calculate the bone levels during 1953 to 1964 of fall-out90Sr in adults in the United Kingdom using the average dietary intake. The results suggested that the average biological half-life of 750 days found in this study is too short for predictions over long periods. Use of a biological half-life of 2,000 days gave results which were very similar to those from the integrated power function, but which were about half the observed bone levels in the national survey. A possible reason for this is discussed.