Particularly in postmenopausal women, peripheral aromatase appears to be the major source of oestrogens which may encourage the growth of hormone-dependent tumours. Studies have therefore been undertaken to determine factors which influence biosynthesis of oestrogens within breast tissues. Aromatase activity was measured in (i) breast cancers by incubating tumour homogenates with [7 alpha 3H]testosterone and characterizing the production of radioactively-labelled oestradiol and (ii) breast fat by incubation of sub-cellular fractions of fibroblast cell lines with [1 beta 3H]androstenedione and monitoring the formation of 3H2O. Evidence has been presented that (i) certain treatment regimes for cancer profoundly influence aromatase activity in breast tumours, (ii) aromatase activity is elevated in breast fat associated with malignancy and (iii) breast-derived fluids and extracts can markedly affect aromatase activity in cultured fibroblasts of breast fat.