The clinical utility of a newly-developed radioimmunometric assay kit for tissue polypeptide antigen (TPA) was evaluated focusing on the serum diagnosis of gastrointestinal cancers. Serum TPA concentrations of normal subjects, patients with various benign and gastrointestinal malignant diseases were measured using this kit. From the TPA concentrations of normal subjects, the cut-off value was determined to be 70 U/l, which was 40 U lower than the previous kit. It showed a fairly high false positive rate in patients with liver dysfunction, so care should be taken in evaluating patients with liver disorders. It showed high positive rates in patients with wide range of gastrointestinal cancers, confirming the wide-range usefulness of this assay. Cancer patients with advanced stages and recurrences showed a higher positive rate and also higher serum TPA concentrations than in the early stages, indicating that this TPA assay is especially useful in the evaluation of therapeutic effects and in the detection of recurrent diseases.