AbstractUsing the cytokinesis‐block technique, lymphocytes from healthy volunteers (n = 9) were evaluated for (1) the radiation dose‐response curve for micronuclei (MN) expression; (2) technique variables on the yield of MN; and (3) the shortest lymphocyte incubation time required for the MN assay. We found that the best fitting of relationships between increasing MN production and increasing irradiation dose (0–4.0 Gy) was the linear‐quadratic model as expressed by the yield equation Y = C + αD + βD2(P= 0.0003). When lymphocytes were irradiated in vitro with 2.0 Gy and harvested at various time intervals, MN increased during the entire 84 hr culture time. The radiation caused a division delay in lymphocyte as indicated by an increased frequency of mononucleated cells and a decreased number of mitotic indices. The data showed that a shortened culture time (60 hr) for the MN assay is possible and that binucleated cells with ⩾ 3 MN were found only in cells irradiated at ⩾ 2.0 Gy. These findings suggest that scoring of MN in lymphocytes may be a practical biological dosimeter for the rapid screening of accidental radiation exposure victims, especially when their clinical manifestations are not obvious. © 1994 W