We evaluated the usefulness of the BEKI TPS radioassay kit as a one-step immunoradiometric assay (IRMA) utilizes monoclonal and polyclonal antibodies specific for serum tissue polypeptide antigen (TPA). This IRMA was found to be highly sensitive to serum TPA; minimum detectable concentration of TPA was 20 U/L. There were no problems in the intraassay and interassay reproducibility and recovery test. However, dilution test of the patient's serum with higher TPA concentrations showed no linear relationship between TPA concentrations and diluted serum samples. The antigen measured by this IRMA was immunologically similar to TPA, and the TPS concentration was closely correlated (r = 0.835, p < 0.01) with the TPA concentration in 101 patient's serum. Four out of 77 healthy subjects (5.2%) and 24 out of 74 patients with benign diseases (32.4%) showed a serum concentration over cut-off value of 71 U/L. The serum TPS concentration was elevated in 80 of 232 patients with malignant diseases (34.5%) including 21 of 26 with hepatocellular carcinoma (80.8%) and 9 of 15 with cholangiocarcinoma (60.0%). In addition, the serum TPA level during the clinical course of patients with malignant diseases was a very useful indicator for the effect of treatment. Thus, our findings suggested that BEKI TPS IRMA kit is a useful assay system for serum TPA as a tumor marker that can be performed by a simple assay operation within about 2 hours.