Table 7. Comparative concentrations of Po?!® in tissue of United Kingdom residents. Tissue Placenta Liver Po?" concn. (pc/100 g) Concn. ratio tissue/plancenta 0.33 1.69 5.1 Kidney 1.72 5.2 Lung 0.54 1.6 Testis Bone 0.39 2.9 1.2 8.8 In order to investigate the possibility of a correlation between natural Po?! and artificial Cs'*7 levels in human tissues, y-Spectrometric measurements have been made of the Cs!** contents of some of the placentas in the Cana-: dian series. The results (Fig. 1) yield a correlation coefficient of 0.93, significant at the O.1l-percent level, for placental concentrations of the two nu- clides and thus provide new evidence for an origin and route of uptake of the polonium isotope that are similar to those of Cs!" The explanation of this finding seems to lie in the natural oon atmospheric content of Rn?*?, whose decay results ultimately in production of Po7!® (Cs!%" is also produced in the atmosphere by radioactivity decay of a rare gas, the fission product Xe!*7), and in the predominant importance of a food chain involving animals dependent for grazing on large areas of slow-growing vegetation that is known to accumulate both nuclides effectively, following their atmosphere. deposition from the C. R. Hite Institute of Cancer Research, Belmont, Surrey, United Kingdom References and Notes 1. C. R. Hill, Nature 208, 423-28, (1965). 2. R. ¥. Gsborne, W. V. Mayneord, in The Natural Radiation Environment, J. A. S. Adams and W. M. Lowder, Eds. (Univ. of Chicago Press, Chicago, 1964), p. 395. 3. I thank the staffs of the various Canadian hospitals and St. Helier Hospital, Carshalton, for providing the samples of placenta, and P. M. Bird of the Canadian Department of National Health and Welfare for organizing the supply of specimens from Canada, The Cs" measurements were made by R. Elrick in our department. 17 January 1966 a