Human breast secretions as collected by nipple aspiration have been analysed for dehydroepiandrosterone sulphate by radioimmunoassay. All secretions collected from non-lactating normal women contained remarkably high levels of dehydroepiandrosterone sulphate as compared with plasma taken at the same time. Although there was a large range of concentrations, levels were of a similar magnitude in different ducts from the same breast and in different breasts from the same individual. No significant difference was detected between secretions from pre and postmenopausal women. Sequential sampling of breast secretions through the menstrual cycle in two normal premenopausal women showed no cyclic variation in dehydroepiandrosterone sulphate concentration. There was also no significant difference between levels in breast secretions obtained from normal women and patients with either malignant or benign breast disease. Analysis of secretions from tumour and non-tumour bearing breasts in cancer patients failed to show consistent differences although, in contrast to normal women, the variation between breasts in individual patients was often marked.