Oestrogen receptor content, lactoferrin, hexokinase, and glucose-6-phosphate dehydrogenase levels were measured in cytosol from 25 primary breast cancers and 3 fibroadenomas. Both hexokinase and glucose-6-phosphate dehydrogenase activity were higher in malignant tissue as compared to benign breast lesions. Oestrogen receptor concentration and lactoferrin content failed to predict the development of metastatic disease, while glucose-6-phosphate dehydrogenase activity was significantly higher in cytosol from those tumours which subsequently metastasized compared to those which remained localized.