An immunoperoxidase study of HBsAg and HBcAg was carried out on liver biopsies of 30 children with chronic hepatitis or idiopathic cirrhosis. Presence of HBsAg and HBcAg occurred only in children who were HBsAg positive in their serum. The number of cytoplasmic HBsAg positive hepatocytes appears higher in children whose liver disease is histologically less severe. The number of hepatocytes with nuclear HBcAg seems directly related to the presence or absence of HBeAg in the serum. Those results are in agreement with previously published observations suggesting that the host's immune response is an important factor of the severity of the liver disease in hepatitis B infections.