APPLICATION OF THE ARTIFICIAL RUMEN AND SIMULATED BOVINE GASTROINTESTINAL FLUIDS PROCEDURE IN THE STUDY OF THE BIOAVAILABILITY OF TRANSURANICS J. Barth Environmental Monitoring and Support Laboratory U.S, Environmental Protection Agency Las Vegas, Nevada ABSTRACT An artificial rumen and simulated abomasal and intestinal fluids procedure was used to study the alimentary availability of plutonium-238. When Plutonium-238 was administered as plutonium nitrate, 10.1% remained soluble following the artificial rumen incubation period and 15.3% following the abomasal period; 30.1% and 32.7% remained soluble when the fluid was held at pH 4 and 5, respectively, during the duodenal phase. The solubility increased to 60.1% following the addition of bile and enzymes with adjustment of the pH to 6 to simulate the jejunum. The increase in plutonium solubility in the simulated jejunal fluid was found to be due to the presence of bile. Plutonium administered as a citrate-buffered plutonium selution was 9.0% soluble following the rumen incubation period, 13.1% following the abomasal period, and 22.5% and 24.8% when held at pH 4 and 5, respectively, in the duodenal phase. The solubility increased to 59.6% following the addition of bile and enzymes with adjustment of the pH to 6. Plutonium administered as 0.06-ymplutonium dioxide spheres was 1.5% soluble following the rumen incubation period, 2.3% following the abomasal period, and 3.6% and 3.9% when held at pH 4 and 5, respectively, in the duodenal phase. Solubility increased to 7.4% following the addition of bile and enzymes with adjustment of the pH to 6. Rumen contents of cattle grazing on plutonium-contaminated desert vegetation at the Nevada Test Site, Area 13, were collected quarterly and incubated in simulated bovine gastrointestinal fluids to study the alimentary availability of field-deposited plutonium-238, plutonium-239, and americium-241. Results to date indicate that the highest concentrations of plutonium and americium in the rumen contents occurred during the late summer or fall. This coincides with the reproductive or "woolly" phase of Eurctia lanata (winterfat), which made up a considerable portion of the vegetation eaten. The solubilities of plutonium and americium were highly variable according to the season and to the predominant vegetation ingested. In most cases plutonium-238 was more soluble than plutonium-239. The solubility of americium-241 was similar to that of plutonium-239. The application of in vitro solubility to predict tissue retention and milk secretion 1s discussed.