Glutathione-based processes are believed to be important determinants of resistance to cancer chemotherapy, and measurement of glutathione (GSH) in tumor tissue is therefore of clinical relevance. Flow cytometric methods have been developed for measuring cellular GSH content that appear to correlate well with biochemical determinations. These flow cytometric techniques are rapid, allow tumor cells to be distinguished from stromal elements, and show considerable intratumoral heterogeneity in human tumor cell GSH content. Although large prospective studies are required to determine the correlations between GSH content and treatment outcome in cancer patients, this approach is a powerful alternative to standard biochemical assays for GSH.