PLATES XLIV-XLVII)Srs9 and 81-90 are both products of uranium fission in a chain-reacting pile. Although the fission products are small in total mass, their radio-activity makes them potentially dangerous. Radio-active strontium is known to produce osteogenic sarcoma in rabbits, rats and mice in relatively small doses.Only traces of strontium are present in normal mammalian tissues and little is known of its behaviour in the animal body, though it is suggested (McCance and Widdowson, 1939) that it behaves physiologically like calcium. Early experiments with radio-active strontium Pecher and Pecher, 1941) showed that, by whatever route it is absorbed, it is laid down primarily in bone. This has been confirmed by much recent work (Hamilton, 1947 ;Copp et al., 1947 ;Lisco et al., 1947).In the experiments here described, the solutions of strontium isotopes as supplied by A.E.R.E., Harwell, contained varying proportions of a SrEs, SrsO and YsO equilibrium mixture. SrE9 has a half life of 53 days (Seaborg and Perlman, 1948), breaking down by 8-particle emission to stable YES. SrgO has a half life of 25 years (Seaborg and Perlman) and breaks down by 8-particle emission to YsO, which is itself radio-active, having a half life of 62 hours, breaking down by 8-particle emission to stable ZrsO. The proportions of radioactive materials in the solution were therefore constantly changing, and towards the latter period of the work the solution contained as much as 45 per cent. each of SrsO and YsO. Workers elsewhere have also used mixtures of SrEs and SrsO, but have had available freshly radio-active strontium, i.e. SrE9, with a very small admixture of SrsO : they have felt justified, therefore, in ignoring the possible * Work done on behalf of the Protection Sub-Committee of the Medical Research t Nuffield Graduate Assistant. Ibid., xlvi, 91. iii, 164. xlvi, 86. J . Biol. Chem., cxxxix, 433. Rev. Mod. Physics, xx, 585. Biochem. J . , xliv, P. xxxv. Brit. J . Radiol., xxi, 347. Strahlentherapie, lxxiii, 509.