The appearance and clinical significance of an atypical fraction of serum alkaline phosphatase called "biliary" have been evaluated in patients with primary liver cancer (PLC) and control subjects. The "biliary" fraction was present in all 42 PLC patients and proved to be the most sensitive of the biochemical parameters utilized. The association with alpha-fetoprotein determination allowed us to identify patients affected by hepatic pathology of neoplastic type, with a diagnostic efficiency of 92%.