To evaluate the clinical significance of serum tissue polypeptide antigen (TPA) levels in patients with malignancy, serum TP A levels were measured by radioimmunoassay in 49 normal controls, 9 patients of postoperative colon cancer without recurrence and 68 patients with various untreated malignancy, who visited Seoul National University Hospital from February, 1983 to September, 1983. The results were as follows; 1) Serum TPA levels in 49 normal controls were in the range of 22-135 U/L $(74{pm}28U/L,;mean{pm}S.D.)$. There was no sex or age difference. Normal upper limit of serum TPA was defined as 130 U/L (mean+2S.D.). 2) Serum TPA levels in 68 patients with various untreated malignancy (stomach cancer 33 cases, colon cancer 11 cases, lung cancer 10 cases, primary liver cancer 9 cases and metastatic cancer of unknown primary site 5 cases) were in the range of 10-800 U/L $(189{pm}170U/L,;mean{pm}S.D.)$ and significantly elevated, compared with those of normal controls (p<0.005). 3) The sensitivities of serum TPA in various untreated malignancy were 39% in stomach cancer, 55% in colon cancer, 50% in lung cancer, 67% in primary liver cancer and 80% in metastatic cancer of unknown primary site respectively. 4) The sensitivities of serum TPA related to resectability in stomach and colon cancer were 32% in resectable stomach cancer, 50% in unresectable stomach cancer, 29% in resectable colon cancer and 100% in unresectable colon cancer respectively. 5) The mean value of serum TPA levels in 9 patients of postoperative colon cancer without recurrence was $70{pm}39U/L$ and significantly decreased, compared with that of untreated colon cancer, $180{pm}150U/L$ U/L (p<0.05). 6) In patients with stomach or colon cancer, there was no significant correlation between serum TP A and serum CEA levels, but simultaneous measurement of serum TPA and serum CEA levels increased sensitivities. From above results, we concluded that serum TPA level is a useful indicator reflecting tumor activity and responses to anticancer treatment in patients with malignancy.