A modification of the radioimmunoassay procedure for measuring antibody to hepatitis-associated antigen (HAA) using a double antibody precipitation method in a microtiter system is described. The test is 2,000 to 500,000 times more sensitive than complement fixation and is reproducible and specific for anti-HAA. The frequency of anti-HAA was 14.4% in voluntary blood donors, 22.6% in commercial blood donors, 14.8% in blood bank personnel, 11.4% in laboratory personnel and 82.6% in persons given multiple transfusions. The high frequency of anti-HAA in persons with no history of hepatitis or parenteral exposure to blood suggests that HAA-positive hepatitis is endemic in the United States and that it is transmitted by a non-parenteral route.