This report describes immunocytochemical and ultrastructural methods which led to the identification of Reovirus type 3 (Reo‐3) in the porta hepatis of a patient with extrahepatic biliary atresia. The study indicates that Reo‐3 antigenic sites are demonstrable by the avidin‐biotinylated complex peroxidase method following formalin fixation and paraffin embedding, but are destroyed by freezing and thawing prior to fixation. Deparaffinization of the block and subsequent rembed‐ding in epon‐araldite did not alter immunoperoxidase staining. This procedure offered the advantage of higher light microscopic resolution of semithin (1 jtm) sections and assisted in the selection of specific areas for ultrastructural studies.

Localization of Reo‐3 in extrahepatic biliary atresia was confined to a biliary remnant in which there were acutely inflammed, partially necrotic microscopic ducts. Electron microscopic examination of the immunoreactive sites revealed virus‐like particles similar in appearance to Reo‐3 particles in infected tissue culture cells. The observations presented here support previously reported serologic data which have shown an association between Reo‐3 infection and extrahepatic biliary atresia.