Serum galactosyltransferase activity (GT) has been studied using isoelectric focusing chromatography in normal subjects, in patients with non-neoplastic liver diseases, and patients with liver neoplastic diseases (hepatoma and liver metastases) using isoelectric focusing chromatography. Freshly obtained serum was applied to an isoelectric focusing column containing Ampholine (pH 4-6) and the fractions were assayed using asialo-agalactofetuin as the galactose acceptor. Three peaks of GT activity were found in sera from normal subjects at pH 4.8, 4.95, and 5.1. In contrast, sera from patients with hepatoma or with gastrointestinal adenocarcinoma with liver metastases contained only two peaks of GT activity (pI 4.75 and 4.95). Sera from patients with non-neoplastic liver diseases had similar GT isoelectric focusing patterns as those from normal subjects. These results suggest that the isoelectric focusing patterns of serum GT activity may be useful in characterizing human liver neoplasia.