In an attempt to clinico-pathologically examine asymptomatic HBs AG carriers, follow-up studies were made on 57 HBAg-positive blood donors with the S-GPT within normal range and the following conclusions were arrived at: 1) The results of liver function tests made in the present studies revealed the following rates of abnormalities: the S-GPT was abnormal in 14.5% of the subjects, the S-GOT was abnormal in 9%, the serum total bilirubins were abnormal in 12.2%, the serum alkaline phosphatase level was abnormal in 24.5%, the TTT was abnormal in 4.4%, the ZTT was abnormal in 2.2%, the gamma-globulin was abnormal in 21.2%, and the ICG retention was abnormal in 25.6%. It was thus necessary to make a follow-up study of the results of liver function test. 2) Anti-HBs was negative in all subjects, the rate of lymphocytic blastogenesis in the peripheral blood (tested by the addition of PHA) was low in 7 (36.8%) of 19 patients, and the MIF test by the addition of the purified HBs Ag revealed that 17 subjects, excluding one subject with a histologic picture of acute hepatitis, were not susceptible to HBs Ag. It was, therefore, surmised that immunological insufficiency would be involved in the development of asymptomatic carriers. 3) Histologic examinations, made on 20 subjects, revealed A.V.H. in one subject, N.S.R.H. in seven, N.S.R. in ten and fatty liver in two, and further revealed mild, diffuse inflammations in 8 subjects in the first two group (40% in total). Further, pleomorphism was noted in the hepatocytes of 8 (40%) of these 20 subjects, and a study is under way of the significance of the pleomorphism.