developed science, and extremely sensitive analyses exist for all forms of internal contamination. The knowledge of retention and excretion functions is fairly well-accepted, and is certainly much better established than at the time of this incident. However,if pooror limited data exist, the use of these models will produce results with large uncertainties. For instance, the use ofa single sample of pooled urines obtained 15 days. after the initial intakes, will have a very poor predictive potential. If periodic samplingis conducted for individuals beginning shortly after exposure to catch short lived radionuclides and characterize early clearance, analyses of internal doses can be made with good confidence. Long term follow-up, by continued in-vivo counting can also result in characterization of retention patterns and dosimetry of radionuclides sequestered in the thyroid or skeleton that are reasonably accurate. 6.2 Radiation Dosimetry: Biomarkers Several biomarkers are currently in use or under investigation as tools for estimating biological dose in populations exposed to ionizing radiation (38). Among these are assays that detect chromosome damage and somatic mutations at the hprt orHLA-A locus in peripheral blood lymphocytes and two assays that detect mutations induced in erythroid stem-cells that are subsequently expressed as variants or mutations in erythrocytes (i.e., hemoglobin variants or mutations at the glycophorin-A [GPA] locus). Each of the assays for mutations has advantages and disadvantages. For example, GPA analyses are automated and can be rapidly accomplished on large numbers of blood samples. However, mutations are only detected in heterozygotes, the assay requires several weeks post- exposure for expression of stem-cell mutations, andit is not possible to produce in vitro dose response curves as calibration standards. By far the mostsensitive biological method that is currently available for estimating wholebody radiation dose soon after exposure is cytogenetic dosimetry using radiation-induced chromosomeaberrations in cultured lymphocytes as the biomarker of exposure. The Bravo accident occurred somesix years before techniques for the culture of lymphocytes werefirst published and a full decade before the first suggestion that radiation-induced chromosomeaberrations could be used as a biological dosimeter to estimate absorbed dose 39