Fission products and the dairy cow. 2. Some aspects of the metabolism of the alkaline-earth elements calcium, strontium and barium.

Radioactive isotopes of strontium (59Sr and '*Sr) are generally regarded as the most dangerous of the products of nuclear fission. The passage of radiostrontium deposited on soil and vegetation into the human diet has therefore received considerable attention. Since strontium is similar to calcium in its metabolic behaviour it has become common practice to study the simultaneous behaviour ofthe two elements in the various physiological processes in plants and animals which are involved in these transfer processes. In this way it has been demonstrated that discrimination against strontium in favour of calcium occurs, for example, in their passage from the diet of a dairy cow to its milk (Comar & Wassernan, 1958). 14'Ba is also a constituent of mixed fission products and is present in significant amounts in tropospheric fall-out but, because of its relatively shorter half-life (12.8 days as compared with 54 days for 89Sr and over 20 years for 90Sr), it is considered to present a smaller hazard to the human and animal population than the strontium isotopes. Distribution studies in human tissues (Sowden, 1958) suggest that the behaviour of 140Ba in the animal body differs from that of strontium and calcium and, in fact, may be more closely related to that of radium. The present paper describes studies on the comparative behaviour of 45Ca, 89Sr and 140Ba in the dairy cow. These experiments were designed particularly to yield information on the physiological processes involved in the discrimination between the three elements during their transfer from diet to milk.