As an approach to identifying specific cellular markers for the cyto-toxic action of x rays in mammalian cells, we used the QUEST system of high-resolution, two-dimensional protein gel electrophoresis and a computerized data base on proteins to score quantitative changes in patterns of protein synthesis. We measured the responses elicited after x irradiation of cells from the normal rat cell line REF52 as well as two oncogene-transformed REF52 cell lines with Ela or Ela plus the mutated c-Harvey-ras T24 (HRAS1 T24) allele. The transformed cell lines differed substantially in the patterns of changes in protein synthesis seen immediately after DNA damage. In addition, we identified a specific subset of growth-regulated cellular polypeptides that are correlated with the observed increase in x-ray-induced cell killing hi the transformed cell lines. One of these polypeptides was cyclin (proliferating-cell nuclear antigen), a cell-cycle-specific DNA polym-erase delta auxiliary factor. Synthesis of this set of coregulated polypeptides was rapidly suppressed by x irradiation in normal REF52 cells only. The inability of x irradiation to induce suppression of protein synthesis in cells from the transformed cell lines correlated with the increased susceptibility to x-ray-induced cell killing. This finding suggests that the cellular processes that underlie regulation of DNA-damage-induced growth arrest at the level of replicative elongation plays a role hi determining the survival of x-irradiated cells. [J Natl Cancer Inst 1988:80:1402–14071