We have examined the effects of ionizing radiation on somatic mutations in vivo, using the hprt clonal assay. The study was performed on blood samples obtained from children exposed during a radiological accident that happened in 1987, in Goiânia, Brazil. The group of children exposed to ionizing radiation includes six males and four females ranging in age from 6 to 14 years at the time of exposure. The radiation doses ranged from 15 to 70 cGy. A Brazilian control group, not exposed to ionizing radiation, was also analyzed under similar conditions. the mean hprt mutant frequency for the exposed group was 4.6 times higher than the control group, although the cloning efficiency from the exposed group was significantly reduced. Linear regression analysis of the mutant frequency and ionizing radiation dose did not show a significant relationship between these two parameters. However, a reliable inverse relationship was demonstrated when the regression analysis was performed with nonselective cloning efficiency and ionizing radiation dose. It was demonstrated that nonselective cloning efficiency diminishes as ionizing radiation dose increases. To correct mutant frequencies for clonal events, the clonal relationship between the hprt mutant clones was examined by T-cell receptor analysis. The majority of the mutants analyzed represented individual clones, thus validating the observed mutant frequencies.