Individuals can be exposed to high doses (more than 5Gy) during radiation accidents. It is, of course, helpful to the physician to have biological indicators also for such high doses. The problem with most cytogenetic indicators is, that the response levels off at doses starting around 5-7Gy of low LET radiation and that the dose-response curve even declines after doses exceeding about 10Gy. Thus, it may be difficult to decide, whether the dose was, for example, 8 or 14Gy. We studied how the micronucleus assay can be used to give information also in the high dose range. It turned out that micronucleus frequency itself cannot be used for the estimation of doses exceeding about 5-7Gy. There are, however, at least three other endpoints that can be determined in the cytochalasin B assay that can assist the decision in the high dose range: (1) the number of mononucleated cells; (2) the ratio of tri- to tetranucleated cells; (3) the average micronucleus frequency in micronucleus positive binucleated cells.