. in irradiated persons. Although it was not possible to employ this technique in studies of the Marshallese in 1954, evaluations of chromosomeaberrations or micronucleiin cultured lymphocytes would be the biological method ofchoice for estimating dose in individuals if such an accident occurred today. During the last three decades, cytogenetic methods have been widely and effectively applied for estimating dose among exposed persons in radiation accidents that have occurred world-wide including the Chernobyl; Goiania and El Salvador accidents. In-vitro dose-response curves have been generated for chromosome aberration induction in lymphocytes exposed to a variety of radiation qualities, and such curves are readily available to serve as calibration standards for comparing with findings in recently exposed persons. When blood samples are obtained promptly after exposure and delivered to laboratories with a minimum ofdelay, cytogenetic evaluations using classical staining for radiation-induced dicentric chromosomes can detect average whole-body doses of about 100 mGy and above in the exposed individual. As newer techniques are being developed, for example, the use of fluorescence in situ hybridization techniques to "paint" chromosomes, combined with automated systems for metaphase location and possibly metaphase analysis, it is possible that the sensitivity of as low as 50 mGy may be achieved in the future. In instances when several dozen to several hundred persons are exposed, single laboratories could not be expected to have the capability for providing timely dose estimates for each individual; however, collaborative efforts between several laboratories in the international community have been successful in the past, as has been demonstrated in several biological dosimetry evaluations in persons exposed during the Chernobyl accident. Similar approaches could be used should a major radiation accident involving large numbers of persons occur in the future. When cytogenetic analyses are employed as biomarkers of dose, consideration should be given to the following issues: 6.2.1 The Limitations of the Technique Evaluationsfodiation-induced chromosomeaberrations in cultured lymphocytes serve as —— ~ ésoe met t* ween 40