The serum levels of the carbohydrate antigen sialyl Lewis X (SLEX) increase in liver diseases (Sunayama T, Okada Y, Tsuji T., J Hepatol 1994; 19: 451-458). However, it is not known whether the increased serum SLEX levels are associated with the increased levels of its carrier molecules and/or the increased density of SLEX per carrier molecule. By using of rabbit antibody against an SLEX-positive fraction from HepG2 culture supernatant, we developed an enzyme-linked immunosorbent assay to determine the serum levels of the carrier molecules of SLEX (CMSLEX). The CMSLEX-levels in patients with hepatocellular carcinoma were significantly higher than those of normal controls (P < 0.001) and benign chronic liver diseases, i.e., chronic active hepatitis, mild and severe form, and liver cirrhosis (P < 0.05). Patients with chronic persistent hepatitis and chronic active hepatitis, mild form, had higher CMSLEX-levels than normal controls (P < 0.05). The serum CMSLEX-levels did not differ significantly among benign liver diseases. We concluded that serum CMSLEX-levels increase nonspecifically in liver diseases. This is a possible molecular mechanism for the increased serum SLEX levels in liver diseases.