The sterilizing conditions of HBs Ag-contaminated materials were studied by means of the inactivation of the antigenicity (infectivity being thought to be lost when HBs Ag is inactivated). HBs Ag-coated beads and the serum from an HBs Ag-positive patient were used. After each sterilizing procedure, the antigenicity was studied by radioimmunoassay. HBs Ag-positive serum is easily inactivated by boiling and by steam under pressure; however, HBs Ag-coated beads require increased steam under pressure. For sterilizing HBs Ag-contaminated materials, 30 minutes at 132 degrees C is recommended.