results of surveys conducted by the United Kingdom Atomic Energy Authority*-", by multiplying the mean concentration in rain from all sites for which quarterly determinations were made by the meanrainfall throughout the United Kingdom. The average ratio of caesium-137 : strontium-90 in fall-out is relatively constant, and the deposition of caesium-137 has therefore been calculated from that of strontium-90 taking the ratio of caesium-137 : strontium-90 as 1-7 up to 1961 and 1:5 since then. The country-wide mean levels of radioactivity in milk were taken from the reports of the Agricultural Research Council Radiobiological Laboratory?*-**, Strontium-90 It has been recognized for many years that strontium-90 may enter milk as a result of both the ‘direct’ contamina- tion of pastures and other forage crops with airborne debris and the absorption of strontium-90 through plant roots from the soil. The magnitude of these two components is related, respectively, to the recent rate of fall-out and the cumulative deposit in the soil. Equations of the following type have, therefore, been widely used to relate the ratio of strontium-90 to calcium in milk to the rate of fall-out and cumulative deposit of strontium-90: C= 9,-F, + paFa (1) where C is the 12-month mean ratio of strontium-90 to calcium in milk (pe./g), 7, is the annual deposit of stron- tium-90 (me./km*/year), /g is the cumulative deposit of strontium-90 at the middle of the year (mc./km!*), and Pr, Pa are described as the ‘rate’ and ‘soil’ proportionality factors. Such equations have been shown to provide a useful basis for predicting both the average levels of con- tamination in milk in a number of countries and also the world-wide situation':5:?1,_ None the less, the relationship between the pattern of fall-out and the contamination of milk is considerably more complex than the equation suggests. The assumption that the soil factor is constant ignores the progressive penetration of strontium-90 deeper into the soil which will reduce its availability to pasture plants*. <A further defect is that the equation takes no account of the fact that an appreciable, though variable, fraction of the diet of cattle consists of stored food grown in the previous year; thus the rate of fall-out at that time will influence the extent to which the current diet of cattle has been directly contaminated by airborne debris. In some areas this ‘lag’ effect of the rate of fall-out may be enhanced due to the absorption by plants of strontium-90 which has lodged in the basal tissues and been retained for several months without being incorporated in the soil. The inadequacy of equation (1) from this point of view is apparent when the mean annual ratios of strontium-90 2 44k