The effects of cellular GSH levels on the cytotoxicity of MNNG and mitomycin C were examined in normal and BSO treated mouse C3H10T1/2 cells. MNNG was less cytotoxic in the GSH depleted cells (less than 10% of normal) whereas the cytotoxicity of mitomycin C was not influenced by thiol status. This is compatible with the alkylating agent MNNG requiring thiols for activation to the methylating electrophile. Conversely, thiols have little if any effect in modulating the activity of mitomycin C. When naturally thiol deficient human fibroblasts were compared with BSO treated fibroblasts depleted to a similar GSH level (less than 10% of normal), only the BSO depleted cells were less effected by MNNG. The GSH deficient cells showed the same MNNG dose response as normal human fibroblasts. These studies indicate that a naturally acquired thiol status and an equivalent induced thiol status need not behave the same and this needs consideration when evaluating the role of thiols in influencing cellular response to chemotherapeutic agents.