Reduced glutathione (GSH) has been demonstrated in benign and malignant human breast lesions using a newly developed histofluorescence technique. GSH was present in every lesion and in each case was localised to the epithelium. A semi-quantitative assessment revealed a moderate amount of GSH in normal epithelium and fibroadenoma and a high level in apocrine metaplasia, epitheliosis and intraduct carcinoma. Invasive ductal carcinoma contained a variable amount of GSH. Correlation between fluorescence intensity and histological grade of ductal carcinomas was almost statistically significant but a relationship to oestrogen receptor status was not detected. The rapid assessment of GSH in breast cancer may aid in the selection of optimum chemotherapeutic regimens.