BOVINE TRANSPORT AND RETENTION OF PLUTONIUM-238 WITH SPECIAL EMPHASIS ON THE GASTROINTESTINAL UPTAKE OF IN VIVO LABELED MILK W. W. Sutton, R. G. Patzer, P. B. Hahn, and G. D. Potter U.S. Environmental Protection Agency Environmental Monitoring and Support Laboratory, Las Vegas ABSTRACT A two-phase experiment was conducted to determine whether in vivo plutonium- labeled milk presents the nuclide in a more biologically available form than the tn vitro plutonium preparations typically administered for intestinal uptake studies. Dairy calves were fed either in vivo plutonium-labeled milk or an tn vitro plutonium-labeled milk prepared through citrate to uncontaminated milk. Plutonium retained in at the time of calf sacrifice, will be-used to compare availability of plutonium in the two treatment groups. not currently available on these calf tissues but will the addition of plutonium the tissues, collected the relative biological Analytical results are be presented later. An additional objective was to examine the excretion patterns and tissue retention of plutonium in the dairy cows used to produce the in vivo plutoniunlabeled milk. A total of six cows received various acute intravenous doses of citrate-buffered plutonium-238 nitrate, and the subsequent transport to milk, urine, and feces is reported. Tissue samples were taken from four of the adult cows, but the plutonium concentration in these samples, along with the above-mentioned calf tissue samples, will be presented later in the comprehensive report. , INTRODUCTION The long physical and biological half-life of plutonium has dictated that considerable effort be devoted to quantifying plutonium transport through the various trophic levels. Results on the absorption, distribution, and excretion 179