There was usually an increase in americium-241 solubility when the pH was adjusted to pH 4.5 to simulate the duodenum. However, the change in americium-241 solubility during the jejunal incubation. period was not consistent. There was a sharp increase in americium-241 solubility in trial 9 and a moderate increase in trial 10 during the lower intestinal incubation period. The high levels of plutonium and americium-241 solubility during trial 9 were accompanied by the lowest concentrations of plutonium and americium-241 in the The rumen contents were taken from a rumen contents (Tables 3, 4, 5, and 6). cow grazing in the outer enclosure. The minimum concentration of plutonium and americium-241 in rumen contents collected from the steers grazing in the inner enclosure occurred during trial 7 (January 19, 1975), while the maximum concentration occurred during trial 6 (November 5, 1974). Analyses of the vegetative composition of the rumen contents of the cattle indicate that Eurotia lanata was the predominant species present during trials 6, 8, 9, and 11. Atriplex confertifolia (shadscale) was the predominant species present during trials 7 and 10 (Table 7). DISCUSSION Since this study is still in progress, all interpretations are subject to reconsideration as additional data become available. The results presented here are in general agreement with the earlier NAEG report of Barth (1975). Since the materials and reactions in the digestive fluids and plant residues form an extremely complex mixture, the term "solubility" refers only to the fraction of plutonium or americium-241 in solution at the termination of a specific digestion phase and disregards all potential intermediate reactions. Particulate dissolution, which is an initial reaction, could be greater than the final solubility following secondary or further reactions. The greater solubility of plutonium-238 as compared with plutonium-239 reported here and earlier by Barth (1975) is consistent with a preliminary report by Smith et al. (1975) that the 239/238-plutonium ratios in tissues are lower than those found in the ingesta, indicating that plutonium-238 is more readily absorbed and retained than is plutonium-239. The similarity of americium-241 to plutonium-239 solubility reported here is not in agreement with Raabe et al. (1974), cited earlier, who reported a greater solubility of americium. The disagreement in results could be due to the different chemical composition of the fludis used, especially phosphate, and perhaps because in this study decay of plutonium-241 had resulted in ingrowth of americium-241 within the plutonium particles. The behavior of americium-241 differed from that of plutonium in respect to response to the jejunal incubation period. There was a consistent increase in plutonium solubility during the jejunal incubation period, while this was not 126