Thiol-containing compounds of low m.w. play a key role in protecting cells from the toxic effects of ionizing radiation, other free-radical-generating reactions, reactive oxygen species and chemical toxins. Previous studies have emphasized the importance of the tripeptide glutathione, which is the most abundant soluble thiol in cells. Cysteine is more difficult to quantitate than glutathione, with reported concentrations only 1-10% that of glutathione in most normal tissues and tissue culture cells. Using an electrochemical method (oxidation of the functional -SH group) which allows the direct assay of thiols after acid extraction of cells or tissue, our measurements confirm the above indicated distribution of glutathione and cysteine in cells and normal tissues. However, in several rat and mouse tumors grown in vivo, we found a much higher proportion of cysteine, sometimes exceeding the millimolar concentrations often found for glutathione. Our results have important implications for predicting tumor radiation resistance since cysteine is a much better radiation-protecting agent than glutathione. Since thiols and oxygen have interacting and opposite effects on the net radiation response, high cysteine levels would directly increase the proportion of radio-resistant cells in tumors.