Expression of cancerous isoenzymes in various malignant tumors has been extensively studied using different biochemical techniques. Cancerous alkaline phosphatase (ALP) isoenzymes, such as placental ALP and variant ALP, are detected from sera of patients with various malignant diseases. Variant ALP is a specific tumor marker for hepatocellular carcinoma (HCC). The positive rate of each ALP isoenzyme is, however, low except for a high positivity of placental ALP in seminoma. Novel gamma-glutamyltranspeptidase (gamma-GTP) isoenzyme is specifically found in about 60% of the sera from patients with HCC but in only 3% of those from patients with benign liver diseases, suggesting a high specificity for HCC. Thus, cancerous isoenzymes are available for diagnosis of various malignant tumors.