The ability of deuterium oxide (D2O) to protect a heat-sensitive and thermotolerance-impaired Chinese hamster ovary (CHO) mutant cell line, HS-36 (Harvey and Bedford 1988), from heat killing was examined and compared to the parent CHO 10B cell line (WT). Both non-thermotolerant (NT) and thermotolerant (TT) G1 populations were examined. D2O differentially protected the NT cell lines from heat killing, with thermal protection ratios (D0) of 2 x 5 and 4 x 3 for HS-36 and WT cells, respectively. D2O provided additional protection to TT cells, but now protected the TT HS-36 cells more than the TT WT cells when the thermal protection ratios of TT cells are compared with those of NT cells (1.15 versus 0.82). The differential protection from heat of the mutant and wild-type lines by D2O may be useful in studies of induced lesions in proteinaceous cellular systems (e.g. the nuclear matrix, cytoskeleton and plasma membrane) using these two paired cell lines.