[en] A study was made to evaluate the sensitivity, precision and accuracy, and practicality of isotope dilution mass spectrometry (IDMS) for bioassay of uranium, plutonium, and thorium in human urine. The study showed that uranium at a concentration of 0.06 μg/L (0.04 pCi/L natural uranium), plutonium at 3 pg/L (0.2 pCi/L Pu-239), and thorium at 0.1 μg/L (0.01 pCi/L Th-232) could be measured with an uncertainty (RSD) of 10% using 10 ml samples. The lower limits of detection for uranium and thorium were set by background contamination, whereas the detection limit for plutonium was determined by chemical yield and intrinsic instrumental sensitivity factors. Precision and accuracy is excellent (approx. 1 to 3%, RSD) at concentration levels where background contamination is insigificant and instrumental sensitivity is adequate. Comparison of IDMS with other methods shows the technique is more sensitive than conventional fluorometric methods but is similar in sensitivity to alpha-radioactivity measurement methods that utilize large sample volumes (1 L). Costs for urine analysis by IDMS ($60 to $100 per sample) are estimated to be considerably higher than cost for fluorometric analysis and approximately the same as the cost for alpha-radioactivity methods. Other methods that have been used or are currently under development are discussed. 100 references, 2 figures, 10 tables