FINDINGS TO DATE The presently available data indicate that the qualitative response in tn vitro bovine gastrointestinal tract of field-ingested plutonium, of uncertain chemical form, was in most cases similar to that of the plutonium-238 nitrate, plutonium-238 citrate complex, and plutonium238 dioxide used in earlier in vitro studies. Increased plutonium solubility was generally noted when the pH was changed from 3.0 to 4.5. These respective pH values characterize a primary chemical difference between. abomasal and duodenal contents. This was followed by a further increase after the addition of bile and enzymes and adjustment of the pH to 6.0 to simulate the upper jejunum. This increase is attributable to the action of bile. In most cases, there was a considerable quantitative difference in the solubility of field-ingested plutonium and plutonium-238 dioxide used in earlier studies. Field-ingested plutonium-238 was found to be generally more soluble than field-ingested plutonium-239 in these fluids. The differences were, however, quite variable. During the late summer, fall, and winter trials, low plutonium solu- bilities were observed, accompanied by comparatively high concentrations of plutonium in the rumen contents. An analysis of the vegetal composition of the rumen contents indicated that this was associated with high levels of Furotta lanata (winter fat). The minimum and maximum solubilities in the simulated duodenum, which is presently believed to be the most important stage, were 0.49 and 13.5% for plutonium-238. For flutonium-239, they were 0.14 and 0.76%. During the spring and midsummer trials, a large increase in plutonium solubility was observed, which was accompanied by a marked reduction in total plutonium concentration in the rumen contents. Analyses of the vegetal composition of the rumen contents indicated a reduction in the proportion of Eurotta lanata and an increase in the proportion Of Oryzopsts hymenotdes reltail grass). (Indian ricegrass) or Sttanion jubatum (squir- The minimum and maximum solubilities in the simulated 37