Two neonates, one with extrahepatic biliary atresia and one with cystic fibrosis, and a 9-year-old child with atresia of the common bile duct had conjugated hyperbilirubinemia and elevated rubella HAI titers when kaolin pretreatment of serum was used. A beta-lipoprotein fraction of the serum that is frequently found in association with biliary obstruction was shown to be the probable source of the rubella HAI inhibitor. This beta-lipoprotein was not removed by standard kaolin treatment of serum, but was removed almost completely by dextran sulfate--calcium chloride treatment. In the presence of conjugated hyperbilirubinemia, routine kaolin pretreatment of serum is an inadequate measure for the removal of interfering substances, as false-positive rubella HAI results are obtained consistently.