We have already reported that the alpha/beta ratio of the cell survival curve could be estimated from the micronucleus frequency in cytokinesis-blocked cells treated with cytochalasin-B after irradiation. In this paper, we investigate the direct relationship between the alpha value and the appearance of micronuclei. Cells of the SCCVII, RIF-1, EMT6, V-79, CHO, HeLa and human esophageal cancer cell lines were used for the study. Low-dose-rate irradiation was used to determine the alpha component of the relationship between dose and micronucleus frequency according to the linear-quadratic (LQ) model. A reduction of the dose rate from 3.09 to 0.0142 Gy/min correspondingly decreased the micronucleus frequency; however, the fraction of binucleate cells without micronuclei was not affected in SCCVII and RIF-1 cells. When this fraction was defined as the normal nuclear division fraction, it decreased exponentially as a function of radiation dose. Then dose vs normal nuclear division fraction (NNDF) was fitted as follows: -ln NNDF = aD + C, where D is radiation dose in grays and C is constant. The slope of the dose vs normal nuclear division fraction was not affected by dose rate. The correlation was also explored between the slope (a) and the alpha value of the cell survival curve determined by the colony formation assay in cells of eight cell lines. These two values showed extremely high agreement: alpha = 1.01 a + 0.00795 (r = 0.99, P < 0.01). This assay was applied to estimate the alpha value of the cell survival curve of human esophageal cancer cell lines established from surgical specimens.