proceed for 3 hours. The contents of the flasks were then converted to simulate the duodenum by the addition of NaOH to adjust the pH to 4.5, and were incubated for approximately 10 minutes. The upper jejunum was simulated by the addition of bile, pancreatin, trysin, and erypsin and adjustment of the pH to 6.0, followed by a 2-hour incubation period. The lower small intestine was simulated by adjustment of the pH to 7.5, followed by incubation for 2 more hours. Following each incubation period, one of the digestion flasks was selected, and the entire contents were separated into solid and liquid fractions by preliminary filtration through several layers of cheesecloth. The resulting filtrate was centrifuged and the supernate collected while the sediment was added to the solid fraction. The entire solid and liquid fractions were analyzed for plutonium-238, plutonium-239, and americium-241 by LFE Environmental Analysis Laboratories Division, Richmond, California (Major et al., 1975). Since a 100% recovery of the liquid phase is not possible, "semiquantitative." this procedure is RESULTS During trials 6, 7, 8, 10, and 11, the rumen contents were collected from fistulated steers allowed to graze in the inner enclosure. During trial 9, the rumen contents were collected at the time of slaughter from a cow which had grazed in the outer enclosure. The percentages of soluble plutonium-238 and plutonium-239 ingested by grazing cattle in simulated bovine abomasal and intestinal fluids are shown in Table l. These data indicate a moderate increase in the solubility of plutonium in the rumen contents collected during trials 7 and 1C and a very large increase in trial 9. In most cases, plutonium-238 was more soluble than plutonium-239. The ratios of the percentage of soluble plutonium-238 to soluble plutonium-239 varied with a range of 0.83 to 6.0. There was a rise in piutonium solubility when the pH was increased to 4.5 in order to simulate the duodenum. This initial increase in solubility was followed by an additional increase during the jejunal incubation period following the addition of bile and enzymes and adjustment of the pH to 6. There was a sharp increase in plutonium-238 and plutonium-239 solubility in trial 9 and in plutonium-238 solubility in trial 10 during the lower intestinal incubation period. The percentages of soluble americium-241 in these fluids are shown in Table 2. These data indicate a moderate increase in the solubility of americium-241 in the rumen contents collected during trials 7 and 10, and a very large increase in trial 9. The 6.5% americium-241 solubility shown in the jejunal phase of trial 6 is anomalous and is not considered in the interpretation of results. The solubility of americium-241 was generally similar to that of plutonium-239. However, the ratios of the percentage of soluble americium-241 to the percentage of soluble plutonium-239 were variable with a range from 0.18 to 6.1. 123