A novel delivery system was used to study NO-mediated cyto- and genotoxicity in two human lymphoblastoid cell lines, TK6 (wild-type p53) and NH32 (p53-null but isogenic to TK6). The delivery system, which supplied NO and O(2) continuously by diffusion through gas permeable tubing, was found to maintain the NO and O(2) concentrations at constant, predictable values. Cellular rates of NO and O(2) consumption and mass transfer coefficients for the two gases were measured in separate experiments and used to calculate the NO concentrations during exposure experiments. The TK6 and NH32 cells were each exposed to several steady state NO concentrations for varying lengths of time, so that the total dose (area under the concentration-time curve) covered a wide range. End point assays, including lethality, apoptosis, mitochondrial damage, and mutation rate in the thymidine kinase (TK1) gene locus, were performed at different posttreatment times. Control experiments using Ar instead of NO resulted in normal cell proliferation for all exposure times tested (up to 36 h). As compared to those controls, significant cell death, apoptosis, and mitochondrial membrane depolarization were observed in NO-treated TK6 cells, and the TK1 mutation rate was elevated. Of particular importance, toxic effects were observed only when the NO concentration and dose were greater than threshold values of approximately 0.5 micro M and approximately 150 micro M min, respectively. If neither or only one threshold was exceeded, the effects were insignificant; when both were exceeded, total cell survival and the number of nonapoptotic cells both decreased exponentially with increasing NO dose. In general, the NH32 cells were much more resistant to NO-induced damage and death than TK6 cells, demonstrating that p53 status is an important determinant of NO-induced cytotoxicity.