To investigate the effects of γ rays on DNA polymerase β properties and its DNA repair functions before or after γ, rays exposure, DNA polymerase 13 activity, gene expression and mRNA levels in SMMCLTNM hepatomas born on nude mice or the samples of the liver cancer tissues from 15 patients were measured with ³H-TTP incorporation test, immunocytochemistry and cytoplasmic dot hybridization analysis, respectively. Irradiation was carried out with ⁶⁰Co-γ rays at ice bath. It was found that DNA polymerase β activity, gene expression and the amount of mRNA were much higher in hepatoma cells than those in normal hepatocytes (P<0.01). In vitro studies, the enzyme activity both in hepatoma and normal liver cells appeared unchanged within 40 Gy γ -ray exposure. Following whole-body exposure of the nude mice bearing SMMC-LTNM with 2 Gy or 4 Gy of γ rays, DNA polymcrasc β activity in hepatoma increased temporarily at 48 hours postirradiation, and its gene expression seemed more active. The enzyme mRNA increased to 1.76-fold of the control group. 72 hours after exposure, all of these changes returned to normal levels. DNA polymerase β participated in DNA repair synthesis and this effect was different between hepatoma and hepatocytes because there were some biologic differences of the enzyme between hepatoma cells and normal liver cells. These data suggested that DNA polymerase β activity, its gene expression and mRNA level in hepatomas could increased temporarily after γ rays exposure, which may facilitate the cells to repair DNA damages from radiation.