Cells from the L5178Y murine lymphoma subline LY-R are twice as resistant to killing by ionizing radiation than the subline LY-S. In contrast, LY-R cells are more sensitive to killing by H2O2, the effect being more pronounced at 37 degrees C than 0 degree C. Initial DNA damage after H2O2 treatment (both temperatures, 5 min) has been estimated by the 'comet' assay (single-cell gel electrophoresis) and fluorescent halo technique. According to both methods, the initial damage is significantly higher in LY-R cells, particularly that inflicted at 0 degree C. Differences between DNA unwinding and rewinding abilities at pH 9 and 6.9 (estimated by the fluorescent halo technique) point to a considerable difference in pH-9-labile damage between the sublines, as observed previously for x-irradiated cells (Kapiszewska et al. 1992). In contrast to findings with x-irradiated cells, however, after H2O2 treatment this damage is more extensive in LY-R cells than in LY-S cells. Thus, the initial pH-9-labile damage corresponds to the pattern of sensitivity to H2O2 and x-rays. We suggest that this is caused by different proportions of cuprous and ferric ions found in the nuclei of LY sublines and by the different ability of these ions to react with H2O2 and water radiolysis products. The copper/iron ratio in the nucleus is 1.31 in LY-R cells and 4.84 in LY-S cells.