The correlation between cell survival curve and dose response curve of MN frequency following irradiations was studied using cytokinesis-block method. Both dose response curves were analyzed by linear quadratic model, i.e. SF = exp (- alpha D - beta D2) and MN frequency = aD + bD2 + c. A good correlation between alpha/beta and a/b ratios was observed in repeated paired experiments (gamma = 0.97). When the cells were treated with BUdR, alpha-type radiosensitizer, a value in dose response curve of MN frequency increased but b value did not. In 10 renal cell carcinomas, the linear correlations between cell surviving fractions and MN frequencies were observed. When radioresponses of 5 esophageal cancer cell lines were evaluated with MN assay, wide range of a/b ratios was found. These data present that MN frequency assay using CB method is available as a tool of rapid assay of radiosensitivity of cells.