In order to exmaine the mutagenicity of cadmium dichloride the author studied the induction of sister chromatid exchanges in chinese hamster ovary K1 cells which treated with cadmium dichloride at various concentrations. The results obtained were as follows: 1. In cells treated with 10(-4) M cadmium dichloride, a small number of cells were visable but no mitosis was bound. 2. The frequencies of sister chromatid exchanges in cells treated with 10(-5) M and 10(-6) M cadmium dichloride as 10.7 +/- 1.9 and 8.3 +/- 2.1, respectively, were significantly increased for control (6.0 +/- 2.3). (p<0.05). 3. There were dose-dependent relationship between the concentration of cadmium dichloride and frequency of sister chromatid exchanges in cells treated with cadmium dichloride at concentration ranging from 10(-5) to 10(-7) M.