Infection with reovirus 3 (Reo-3) has been suggested as the cause of extrahepatic biliary atresia and idiopathic neonatal hepatitis but confirmation has been lacking. Therefore we have searched for a specific anti-Reo-3 antibody response in the sera of patients with biliary atresia or neonatal hepatitis and for Reo-3 antigens in their hepatobiliary tissues. Sera from 23 infants with extrahepatic biliary atresia, 12 with neonatal hepatitis, 30 age-matched control patients with other liver diseases, and 55 control patients without liver disease were tested by an enzyme-linked immunosorbent assay for total (IgA, IgG, and IgM) anti-Reo-antibodies; sera of infants younger than 6 months of age were tested also for IgM anti-Reo-3 antibodies alone. There was no difference between either total or IgM anti-Reo-3 antibody levels in infants with extrahepatic biliary atresia or neonatal hepatitis and levels in control infants. Reo-3 antigens were not detected in the hepatobiliary tissues of 19 infants (18 with biliary atresia, one with neonatal hepatitis) by an immunoperoxidase method that readily demonstrated Reo-3 in control in fected HEp-G2 cells. Our data do not support a relationship between neonatal liver diseases and infection with Reo-3.