cattle allowed free choice of vegetation growing in a plutoniumcontaminated area, provide an opportunity to study factors affecting the biological availability of the plutonium found in nuclear debris ingested at any particular time. Concurrently, biochemical factors affecting the availability of plutonium, such as abomasal and intes- tinal conditions including pH, bile, and enzymes, can be determined. PROCEDURE Rumen-fistulated cattle are allowed to graze periodically, and nonfistulated cattle permanently, in Area 13 of the Nevada Test Site. Samples of whole rumen contents are collected from the fistulated cattle following a 48-hr grazing period. Samples are collected from nonfistulated animals at time of slaughter. Samples of whole rumen contents are added to digestion flasks with simulated abomasal juice consisting of HCl and pepsin, and the pH is adjusted to 3.0. Abomasal incubation in a water bath at 39.5° C is allowed to proceed for three hrs. The contents of the flasks are then converted to simulate the duodenum by addition of NaOH to adjust the pH to 4.5 followed by incubation for about 10 min. The upper jejunum is simulated by the addition of bile, pancreatin, trypsin, and erypsin, and adjustment of the pH to 6.0, period. followed by a two-hr incubation The lower small intestine is simulated by adjustment of the pH to 7.5, followed by incubation for two more hrs. Following each incubation period, the entire contents of one of the digestion flasks are separated into solid and liquid fractions by preliminary filtration through cheesecloth followed by centrifugation of the filtrate. The solid and liquid fractions are analyzed for plutonium-238 and plutonium-239. 36