Male Wistar rats were exposed to 0, 10, 20, and 30 working level months(WLM) of radon and its progeny by inhalation. DNA damage of rat peripheral lymphocytes induced by radon and progeny in vivo was investigated by using ,3,H—TdrR incorporation assay and alkaline single cell gel electrophoretic technique. The CPM value of rat peripheral lymphocytes from radon exposed groups was decreased significantly compared with the control,while the tail movement of lymphocytes gel electrophoresis was increased significantly, with a dose—effect relationship. The results suggest that radon and its progeny can suppress DNA synthesis of lymphocyte, reduce transformation ability of lymphocyte and damage immune function,which are useful implications in radiation protection against radon toxicity.