regard to plutonium absorption, the minimum and maximum solubilities were 0.49% and 13.5% for plutonium-238. For plutonium-239, they were 0.14% and 0.76%. During the spring and midsummer trials, a large increase in plutonium solubility was observed; this increase was accompanied by a marked reduction in total plutonium concentration in the rumen contents. Analyses of the vegetal compo- sition of the rumen contents indicated a reduction in the proportion of Eurotta Zanata and an increase in the proportion of Oryzopsis hymenotdes (Indian ricegrass) or Sttanion jubatwn (squirreltail grass). The minimum and maximum solubilities in the simulated duodenum were 95.1% and 96.6% for plutonium-238; for plutonium-239, they were 44.5% and 90.02. Boyd et al. (1974) reportedthat samples of a polydisperse aerosol of 24 1am0. treated successively at 300° C and 1050 C were relatively insoluble with a fractional dissolution rate of less than 107" per day in a standard solvent prepared to simulate the chemical constituents of lung fluid. Repeatedly the dissolution rate proved to be inversely related to the total phosphate concentration in the standard solvent. Raabe et al. (1974) collected samples of aerosols present in a glove box during a plutonium oxide and uranium oxide powder mixing operation. Solubility studies were run at 37° C using lung fluid simulant with and without the phosphate component. Preliminary indications were that the cumulative plutonium solubility was higher in the standard solvent, both with and without phosphate, than was observed for laboratory aerosols of *39Pu0,. The cumulative solubility of americium in the phosphate-free solvent was at least two orders of magnitude higher than that of plutonium, but was much less in the solvent with phosphate. MATERIALS AND METHODS The procedure described briefly here is essentially that of Barth (1975). Rumen-fistulated Hereford cattle were allowed to graze periodically in the inner enclosure and nonfistulated cattle were allowed to grave continually in the outer enclosure of Area 13 ot the Nevada Test Site (Smith, 1975). The inner enclosure directly surrounds the ground zero of a high-explosive detonation of an atomic device during a safety test. The outer enclosure, a less heavily contaminated area, completely surrounds the inner enclosure, and most of it is located a greater distance downwind from ground zero than the inner enclosure. Area 13 has been described by Tamura (1975). Samples of whole rumen contents were collected from the fistulated cattle following a 48-hour grazing period. Samples were collected from the nonfistulated cattle at the time of slaughter. Samples of whole rumen contents were added to four Erlenmeyer digestion flasks with simulated abomasal juice consisting of HCl and pepsin, and the pH was adjusted to 3.0. Abomasal incubation in a water bath at 39.5 was allowed to 122 If: