Applied of plutonium by dairy cows have been reported at previous Nevada shing establi (1) by ken underta Ecology Group meetings. These studies were absorpinitial ng followi tissues what portion of plutonium was retained in the and urine, milk, to rted tion; (2) determining the amount of activity transpo the of forms l feces; and (3) observing these phenomena after various chemica ng occurri factor on nuclide were administered orally. Due to the large reducti s with gastrointestinal absorption, the transport and retention characteristic were based on relatively low levels of systemic accumulation. Comparisons should be made on the percentage of plutonium transported to milk and edible bovine tissues following a larger systemic dose. Transport coefficients are frequently valid only under experimentally defined conditions and some reports have addressed factors that can cause variations in plutonium uptake. Ragan (1975) noted an approximately fourfold increase for the gastrointestinal absorption of plutonium-239 citrate in iron-deficient mice. There was also a more rapid translocation of plutonium from soft tissues to bone in these iron-deficient mice. Another problem of potential concern is whether in vivo plutonium-labeled milk represents a more biologically available nuclide form than the various in vitro plutonium preparations typically adminThis question istered to establish gastrointestinal transport coefficients. is particularly relevant in reference to juvenile animals, not only because human infants consume relatively large quantities of milk, but also because of the increased juvenile gut absorption of plutonium in some species (Ballou, 1958). To obtain information on the biological availability of tm vivo plutonium-labeled milk, two closely related studies were performed and these experiments have been outlined in this paper. A basic objective was to compare the gastrointes- tinal uptake of tn vivo and tin vitro plutonium-labeled milk by dairy calves. The definitive phase of these experiments has just been completed and the samples of primary concern, i.e., calf tissues, have not been analyzed. paper should therefore be considered as a progress report. This METHODS AND MATERIALS The experiments were conducted in two phases. A feasibility study (Phase I) used a total of two lactating cows and four calves, while the more elaborate definitive study (Phase II) used four cows and twelve calves. Phase I Phase I was directed primarily toward confirming the approximate quantity of plutonium for two different doses, an oral dose for calves and an intravenous dose for the adult cows. Selection of an appropriate dose for the calves concerned plutonium concentrations needed for the in vivo labeled milk. This tn vivo plutonium-labeled milk would have to contain a sufficiently high nuclide concentration to allow for the subsequent detection of plutonium in selected calf tissues. The second dose requiring confirmation was directly 180