Normal HeLa MR cells were irradiated by ,60,Co γ-ray in the dose range of 0~8Gy and the mutants of hprt gene were cloned in the 6-TG containing selective culture and assayed. Three natural spontaneous mutants and eighteen γ-ray induced hprt mutants were recovered. The mutation spectra of hprt exons in all of them were analyzed by polymerase chain reaction (PCR) using 8 pairs of hprt oligonucleotide primers to amplify hprt exons from cellular DNA extracts. The result showed that in the mutants induced by γ-ray, about 77.8% (14/18) of the hprt mutations were deleted,among which 2 were total deletion, the other 12 were partial deletion,and 22.2% (4/18) ,which did not show noticeable changes in the PCR pattern,were point mutation. The mutation degree of hprt exon deletion was found to be dependent on the irradiation dose. However,about 67% (2/3) point mutants and 33% (1/3) deleted mutants were showed in the natural mutants. Their mutation degree was far lower than that of the γ-ray induced mutants. These r esults might be beneficial for further study of the molecular mechanism of mutation induced by γ-ray irradiation as well as for the estimation of irradiation dose.