The transfer of radioactivity to the milk and meat of farm animals is likely be a major exposure pathway of human populations, following an environmental release of radioactivity. The importance of source-dependent bioavailability in determining absorption from the ruminant gastrointestinal tract of the radiologically significant radionuclides (radiocesium, radiostrontium, radioiodine, and plutonium) is reviewed. The requirements for and suitability of in vitro methods of determining bioavailability for absorption of these radionuclides is also assessed. Radiocesium absorption varies over a 50-fold range, depending upon dietary source. Source-dependent bioavailability is therefore an important factor in determining the radiocesium contamination of ruminant-derived food products, and reliable in vitro techniques have been developed to rapidly determine its bioavailability. In contrast, under conditions of adequate calcium intake, the absorption of radiostrontium will not be greatly influenced by the dietary source. Results of in vitro extractions of radiostrontium could be misleading, as they indicate differences in bioavailability that are not observed in the animal. Absorption of radioiodine is complete and independent of source. There is currently a lack of data on plutonium absorption in ruminants. However, on the basis of the limited data available, in vitro incubations provide a measure of the relative bioavailabilities of plutonium from different sources. The applicability of the techniques reviewed to the study of other radionuclides and heavy metals is discussed.

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The gastrointestinal absorption of 210Po was determined by comparing tissue retention after oral and systemic administration. The results indicate an increase in absorption in adult rats for 210Po administered in liver compared with 210Po nitrate with estimated absorption of 5 and 13%, respectively. For 210Po citrate, values of about 7% were obtained in 1-day-old neonate and adult rats while absorption in guinea pigs was estimated to be about 23% in 1-day-old neonates, 17% in 5-day-old neonates, and 9% in adults. Gut retention of ingested 210Po in neonates was high in rats but not guinea pigs. In adult animals, but not neonates, the liver accounted for a greater proportion of 210Po reaching the bloodstream after ingestion than after systemic injection. The significance of these results is discussed in relation to current assumptions made in the calculation of doses from 210Po.

The gastrointestinal absorption of Fe and Co by rats and guinea pigs of different ages was measured by comparing the whole-body retention of 59Fe and 57Co after oral and intraperitoneal administrations. The age-groups studied included newborn, weanlings and adults. The absorption of both Fe and Co decreased markedly with age in both rats and guinea pigs. In the rat, absorption remained ele vated during the suckling period, while in the guinea pig absorption decreased markedly during suckling. In both species, Fe and Co absorption were similar, and remained elevated above adult values for some time after weaning. The generally greater absorption of Fe and Co by rats than by guinea pigs and the longer duration of maximal absorp tion in the rats may have involved differences in Fe status in the two species and differences in the timing of gut mat uration.

For 6oco y-radiation and electrons of incident energies from 3 t o 3.5 MeV, dose measurements using ionization and FeSO, methods are compared. If factors given by the HP-4 'Practical guide to electron dosimetry' are used, results from the two methods do not differ by more than 4%. However, the ionization results based on the HPB values of C, give a mean G-value of 152 over this energy region. If it is assumed that photon beams producing primary electrons of a given mean energy have the same G-value as electrons of that energy, then electron G-values predicted from photon results using HPA CA values and the same FeSO, equipment are about 8% higher than experimental values for 6 t o 10 MeV electrons.