A glutathione-S-transferase-pi (GST-pi) immunoradiometric assay was devised as a potential marker for serodiagnosis of malignant disease. Elevated serum GST-pi levels were observed in patients with various gastrointestinal malignancies including gastric, esophageal, colonic, pancreatic, hepatocellular, and biliary tract cancers. Patients with benign gastrointestinal diseases had normal GST-pi, but some patients with chronic hepatitis and cirrhosis had slightly elevated levels. Over 80% of patients with Stage III or IV gastric cancer and even about 50% of those with Stage I and II had elevated serum GST-pi. After surgery serum GST-pi levels returned to normal. Resected stomach cancers were immunohistochemically positive for GST-pi. During chemotherapy of colonic, gastric, and hepatocellular cancers with a series of different drugs, GST-pi changed in a biphasic manner; increases during initial phases of therapy may reflect acquisition of drug resistance by the tumor. In general, serum GST-pi assays provide a sensitive and reliable marker for gastrointestinal malignancies.