Nineteen biochemical parameters, most of which have been individually advocated as tumour-index-substances for breast cancer, were measured in 51 patients with breast disease, 42 of whom had active breast cancer. Seven of these parameters were raised in more than half of the 17 patients of the series with overt metastases; these were serum ferritin (88%), C-reactive protein (87%), carcinoembryonic antigen (81%), acid glycoprotein (75%), total alkaline phosphatase (64%), sialyl transferase (56%), andthe urinary hydroxyproline/creatinine ratio (73%). The incidence of biochemical abnormalities in patients in this group compared favourably with the results of physical methods of detecting metastases. 7 of 16 further patients without evidence of distant metastases, but who had a poor prognosis as judged by histology of the primary tumour and axillary lymph-nodes, had abnormalities of at least one of the seven parameters. 3 of these patients have relapsed within a year of mastectomy. The results suggest that these biochemical tests could assist in monitoring metastatic disease and could indicate at the time of mastectomy, patients who might benefit from immediate systemic therapy in addition to local treatment of their breast carcinomas.