Cells from the L5178Y murine lymphoma subline LY-R are twofold more resistant to killing by ionizing radiation than the subline LY-S. In contrast, LY-R cells are more sensitive to killing by hydrogen peroxide. Cells of the two sublines in logarithmic growth phase were treated with hydrogen peroxide in phosphate-buffered saline for 1 h at 4 degrees C or 37 degrees C. From the comparison of D(o) values it followed that at 37 degrees C LY-R were 3.6 times more sensitive to the killing effect of H2O2 than LY-S cells; at 4 degrees C they were 11 times more sensitive. Treatment with hydrogen peroxide at 4 degrees C gave a considerable sparing effect, which was substantially greater for the LY-S subline; for LY-S cells D(o) was 5.7 times lower at 37 degrees C than at 4 degrees C, for LY-R cells only 1.9 times. The mutation frequency (HGPRT) in LY-R cells was increased in proportion to H2O2 concentration and was the same at both treatment temperatures. In contrast, mutation frequencies initially increased, then decreased with increasing H2O2 concentration in LY-S cells treated at 4 or 37 degrees C. The concentration at which the decline was initiated was higher at 4 than at 37 degrees C. DNA damage after H2O2 treatment (both temperatures, 5 min) was estimated from the 'comet' assay (single-cell gel electrophoresis). The initial damage, but not the residual damage, differed significantly in LY sublines. A period of slower repair (between 3 and 10 min) was found in LY-R cells.