Lines irs1, irs2 and irs3, derived from V79-4 hamster cells, are sensitive to DNA-damaging agents including ionizing radiations. However, unlike some other radiosensitive lines, the irs lines show no apparent defect in the repair of DNA strand breaks. We have now assessed the mis-repair of DNA damage in the irs lines by measuring spontaneous and X-ray induced frequencies of mutation in the HPRT gene, irsl was found to be hypermutable, showing instability in spontaneous mutant frequency and an elevation of the radiation-induced frequency relative to the parental line. In contrast, irs2 and irs3 showed similar mutational responses to the parental line. The results support other lines of evidence suggesting that irs1 has a mis-repair phenotype. The irs2 line has previously been shown to have a phenotype similar to cells from the human disorder ataxia-telangiectasia and this similarity is maintained in their mutational response to X-rays. The irs lines were also tested for ability to undergo V(D)J recombination, since this process has recently been found to be defective in some radiosensitive lines with impaired double-strand break repair. Using an extrachromosomal vector containing a V(D)J rearrangement cassette, correct recombination was shown to occur at similar frequencies to parental V79-4 cells in each of the three irs lines. Thus the irs lines indicate that processes other than DNA double-strand break repair also control radiosensitivity, in particular those processes which may affect the regulation of DNA repair.