We recently showed that the DNA repair processes occurring after u.v. light induce double strand breaks (DSBs) in the DNA of normal human fibroblast (IMR-90) but not of xeroderma pigmentosum group A cells. We call these lesions 'repair-induced double strand breaks' (RDSBs), since they only occur in excision repair competent cells. In this work, we asked whether these RDSBs occur in IMR-90 cells at low u.v. doses and whether the RDSBs are themselves repairable. We reasoned that holding open the excision-repair induced gaps by inhibiting nucleotide polymerization after u.v. light with hydroxyurea/ara C or aphidocolin should allow us to answer these questions. The results show that as little as 2.5 J.m-2 of u.v. light induces RDSBs during repair incubation when repair inhibitors are present. This suggests that 'hot spots' of high lesion frequency occur and the overlapping excision in these areas will produce RDSBs. Without repair inhibitors, the frequency of such RDSBs will be much lower; but even one unrepaired RDSB is likely to have profound biological effects. By removing aphidocolin from the medium after incubation of RDSBs, we were able to test the repairability of these lesions. The results show that RDSBs are only partially repairable with between 15 and 40% of the breaks unrepaired at 24 h. Because the lesions are partially repairable they should not always cause toxicity and may be involved in processes such as mutation, transformation, and chromosome or chromatid type aberrations of the sort associated with human tumors.