L5178Y-R (LY-R) and L5178Y-S (LY-S) cells differ in sensitivity to UVC radiation (D,,:2.8 and 9.0 Jim' respectively, for cells exposed in Fischer's medium). We used these cells and a DNA unwinding technique in conjunction with I-P-D-arabinOSyl cytosine to determine DNA strand breaks accumulating as a result of enzymatic incision during DN,4 repair. Following UVC exposure DNA strand break accumulation was observed in LY-S cells but not in LY-R cells. The repair defect in LY-R cells is accompanied by a delayed recovery of [3H]thymidine incorporation.