To evaluate the significance of serum beta2-microglobulin in patients with various liver diseases, serurn Beta2 m levels were measured in 44 cases of normal controls, 32 cases of asymptomatie HBsAg carriers and 134 patients with various liver diseases, by radioimmunoassay using Phadebas Beta2~micro test kits. The following results were obtained: I) The mean level of serum B2 m was 1 39+-0.25mg/l(Mean+-S.D.) in normal controls (1.39+-0.23mg/l in 24males, 1.38+-0,27mg/l in 20 fernales). 2) The seram levels of B2m in patients with various Iiver diseaaes and asymptomatic HBsAg carriers were as follows; 1.400.27mg/l in asymptomatic HBsAg carriers, 2.42+-0.377 mg/l in 45 patients with acute viral hepaitis, 2.10+-0.26 mg/l in 46 patients with chronic persistent hepatitis, 2.60+-0.34mg/l in 23 patients with chronic active hepatitis, and 2.60+-0.49 mg/l in 20 patients with liver cirrhosis. Serum B2m levels of each disease group were significantly higher than that of normal controls(p<0.001). 3) There was significant correlation between the levels of serum B2m and the degrees of lymphocytic infiltration in patients with chronic active hepatitis(p<0.001). 4) Significant correlations were observed between the levels of serum betA2,-microglobuin and serum alanine aminotransferase(r=0.68, p<0.05) and bilirubin(r=0.63, p<0.05) in 15 patients with acute viral hepatitis. In conclusion, the serum beta2,-microglobulin levels were increased in patients with various liver diseases, and it may serve as a new index of liver disease activity.