Two hundred four volunteer blood donors with hepatitis B surface antigen found in their blood were followed for 3 to 44 months. The annual clearance rate of this antigen was 1.7%. Liver enzyme levels (aminotransferase) were elevated in 45 (22.1%) on at least one occasion, in 26 (12.7%) for one month or more, and in 13 for more than six months. Liver biopsies were performed on 17 chronic carriers with normal enzymes and nonspecific histologic abnormalities were found in 14 and mild diffuse hepatitis in three. Seventeen carriers with abnormal enzymes were biopsied, and specimens revealed chronic active hepatitis (CAH) in seven, including two with bridging necrosis and three with cirrhosis. CAH was found in 7 of 26 (26.9%) carriers with abnormal liver enzymes persisting for at least one month and 4 of 13 (30.8%) with abnormal liver enzymes for more than six months.