In 28 patients with transitional carcinoma of the urinary tract, all treated with chemotherapy, serial measurements of serum tissue polypeptide antigen (TPA) were performed and correlated to clinical evaluations of response. At the start of chemotherapy elevated levels of TPA were found in 4 out of 14 patients with T2-4NO-2MO tumours and in 7 out of 14 patients with distant metastases. In most patients with elevated TPA levels who responded to chemotherapy, TPA levels rapidly returned to normal. False positive elevations of TPA were observed in 2 patients. It is concluded that serial measurement of TPA for monitoring disease activity has limited value because of the low sensitivity of TPA, especially for patients with early-stage cancer, and because of the occurrence of false positive results.