Of ten tumour markers measured every 3 months after mastectomy in patients with apparently localised primary breast cancer, plasma levels of alkaline phosphatase, carcinoembryonic antigen, and gamma-glutamyl transpeptidase were the most useful in detecting metastatic disease. With these three tests a "lead interval" of 3 months or more was obtained in about half the 23 patients who developed overt metases. Clinical examination, chest X-ray, and these three markers proved the most useful combination of tests in screening for metastases, since at least one test was abnormal in 46 of 47 patients at the time of the development of metastases as judged by more detailed physical tests.