The effects of BSO (buthionine sulfoximine), a potent radiosensitizing and chemosensitizing chemical, on the incidence of micronuclei in four different cell lines have been studied using the cytokinesis-block (CB) method. The number of micronuclei in cultured human peripheral lymphocytes, Chinese hamster cells and human breast cancer cells were not affected by 0.1—2 mmol/L BSO treatment alone. However, significant increase in the incidence of micronuclei in these cells could be detected when BSO was used in combination with γ-irradiation. Linear relationship between the incidence of micronuclei and the radiation dose was observed.