The relationship between microsomal arylamidase activity, determined by a micromethod, and changes of hepatocytes was studied in biopsy samples taken from 107 patients with normal liver histology and viral hepatic diseases. The activities in healthy carriers and chronic persistent hepatitis patients were significantly greater than those of controls, acute hepatitis and chronic active hepatitis patients, and showed a significant correlation to morphologic criteria. Ultrastructurally, regularly increased smooth endoplasmic reticulum (SER) was found especially in healthy carriers. The arylamidase activity reflects virus-induced changes of the SER in healthy carriers and viral hepatic diseases.