A carbohydrate antigen, sialyl Lewis X (SLEX), is an inflammation-associated liver cell antigen, which is increasingly expressed as histological diagnosis progresses. A solid phase radioimmunoassay was developed to determine the plasma levels of this substance which were found to be elevated in about 70% of patients with liver disease, with no significant differences among disease groups. Although the plasma levels of SLEX were not directly correlated with the degree of hepatic SLEX expression, the abnormal values were only found in cases with hepatic SLEX expression. Cirrhotic patients with and without hepatocellular carcinoma had comparable values. Plasma levels of SLEX decreased significantly in chronic hepatitis patients successfully treated with IFN, but not in those without a favourable clinical response. Plasma SLEX was carried by some macromolecules with chromatographic and buoyant properties of mucin-type glycoproteins, and others of non-mucin type. These observations suggested that (i) the plasma levels of SLEX increase significantly but non-specifically in liver diseases, (ii) liver cells in the inflammatory lesion are probably the origin of the SLEX-active glycoproteins in the peripheral circulation, (iii) both the increased hepatic synthesis and impaired secretion of the SLEX-positive glycoproteins might be related to the tissue expression and plasma levels of SLEX, and (iv) plasma SLEX might be a useful marker to evaluate the activity of inflammatory liver disease in individual patients and to monitor their treatment.