Unscheduled DNA synthesis in mouse and man lymphocytes was measured by liquid scintillation method and cell autoradiography after an ultraviolet exposure of 20 J/m,2, .The level of UDS was distinctly lowered in mouse lymph node and blood lymphocytes on the 3 rd—4 th day after γ-wholebody irradiation. The relationship between inhibitory effect and γ-radiation dose was linear. The average grain count per nucleus in blood lymphocytes of two patients, accidentally exposed to ,60,Co, was also reduced. The results demonstrate that the inhibition of DNA excision repair ability must be taken into account of radiation injury effect on cells. The estimation of UV induced UDS might be used as a radiosensitive biological indicator.