To evaluate the significance of serum $beta_2-microglobulin$ in patients with various liver diseases, serum $eta_2m$ levels were measured in 44 cases of normal controls, 32 cases of asymptomatic HBsAg carriers and 134 patients with various liver diseases, by radioimmunoassay using Phadebas $Beta_2-micro$ test kits. The following results were obtained: 1) The mean level of serum $eta_2m$ was $1.39{pm}0.25mg/l(Mean{pm}S.D.)$ in normal controls ($1.39{pm}0.23mg/l$ in 24 males, $1.38{pm}0.27mg/l$ in 20 females). 2) The serum levels of $eta_2m$ in patients with various liver diseases and asymptomatic HBsAg carriers were as follows; $1.40{pm}0.27mg/l$ in asymptomatic HBsAg carriers, $2.42{pm}0.37mg/l$ in 45 patients with acute viral hepatitis, $2.10{pm}0.26mg/l$ in 46 patients with chronic persistent hepatitis, $2.60{pm}0.34mg/l$ in 23 patients with chronic active hepatitis, and $2.60{pm}0.49mg/l$ in 20 patients with liver cirrhosis. Serum $eta_2m$ 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 $eta_2m$ 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 $beta_2-microglobulin$ 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 $beta_2-microglobulin$ levels were increased in patients with various liver diseases, and it may serve as a new index of liver disease activity.