Traditionally, the dicentric chromosome assay (DCA) has been used to derive biological dose estimates for unknown radiological exposures. While sensitive, this assay requires highly trained evaluators and is extremely time consuming. The cytokinesis-block micronucleus (CBMN) assay has been suggested as an alternative to the DCA, as it is much faster to evaluate samples and requires less technical expertise. In order to validate this assay for triage biodosimetry, dose-response curves were generated for six donors at eight doses of gamma-radiation (0-4.0 Gy). Each sample was evaluated by 12 individuals, among three different laboratories and the incidence of micronuclei was determined after counting 50-500 binucleated cells. This study demonstrated that the CBMN assay was capable of detecting radiation doses >or=1 Gy after scoring only 200 binucleated cells. As such, the CBMN assay may provide a sensitive and reliable technique for deployment as an initial screening tool in a large-scale radiological emergency where large numbers of biological dose estimates are required.