The dose response of the number of micronuclei in cytokinesis-blocked (CB) lymphocytes after in vitro irradiation with y-rays and neutrons in the 5 dose range was studied in a heterogeneous population of 4 donors. One thousand binucleated cells were systematically scored for micronuclei. Measurements performed after irradiation showed a dose-related increase in micronuclei (MN) frequency in each of the donors studied. The dose-response curves were analyzed by a linear-quadratic model; frequencies per 1000 CB cells were (0.31 +/- 0.049)D + (0.0022 +/- 0.0002)D2 + (13.19 +/- 1.854) (r2 = 1.000, x2 = 0.7074, p = 0.95) following y-irradiation, and (0.99 +/- 0.528)D + (0.0093 +/- 0.0047)D2 + (13.31 +/- 7.309) (r2 = 0.996, x2 = 7.6834, p = 0.11) following neutron irradiation (D is irradiation dose in cGy). The relative biological effectiveness (RBE) of neutrons compared with y-rays was estimated by best fitting linear-quadratic model. In the micronuclei frequency between 0.05 and 0.8 per cell, the RBE of neutrons was 2.37 +/- 0.17. Since the MN assay is simple and rapid, it may be a good tool for evaluating the y-ray and neutron response.