Serum concentrations of the placental protein 12 (PP12), measured by radioimmunoassay, were found to be elevated in 15 out of 16 (94%) patients with primary liver cancer, 10 out of 68 (15%) patients with other gastrointestinal malignancies, 5 out of 10 (50%) patients with cirrhosis; and 1 out of 33 (3%) patients with benign gastrointestinal disease. No correlation was observed between the serum levels of alphafetoprotein (AFP) and PP12 in patients with primary liver cancer. Using immunoperoxidase staining, PP12 was localised in malignant hepatocytes and hyperactive nodules of cirrhosis, but rarely in malignant cells of other gastrointestinal cancers. Fetal liver was PP12 positive, while adult normal liver was negative. These results indicate that PP12 has certain oncodevelopmental characteristics.