Serum TPS, CA 15-3 and CEA levels were measured in 121 women with breast cancer. The combination of TPS (which measures tumour activity) and CA 15-3 (which measures tumour mass) had a sensitivity of 72% to detecting metastatic disease (n = 71). All 3 markers could significantly discriminate local versus distant metastatic relapsed breast cancer and TPS was more often elevated in the case of bone metastases. In a total of 46 relapsed patients and 49 situations which required therapeutic changes because of progressive disease, the response to treatment (hormonal or chemotherapy) was recorded together with tumour marker changes (25% +/-). After 3 months of therapy TPS (68%) responded earlier and faster than CEA (38%) or CA 15-3 (49%) with no progression (SD+PR+CR). The correlation with clinical deterioration after 6 months of therapy was 44% for TPS 33% for CEA and 28% for CA 15-3. In patients with bone metastases, TPS in addition to CA 15-3 could be used to monitor therapy.