The level of antibody to AIDS-associated virus (LAV) in sera from patients with AIDS or AIDS related diseases (AIDS related complex; ARC) and Japanese hemophiliacs was studied using indirect immunofluorescence. Titer of anti-LAV antibody in sera from 89 patients with AIDS or ARC ranged between 10 and 40, 960 (median: 1, 280) and that of 83 Japanese hemophiliacs ranged from 20 to 20, 480 (median: 640). The distribution of the level of anti-LAV antibody in hemophiliacs was similar to that of patients with AIDS or ARC, and no correlation between the titer of antibody and the presence of symptoms of AIDS was observed. Our results suggested that hemophiliacs in Japan might have been infected with live LAV rather than been immunized with inactivated LAV by injection of factor VIII or IX concentrate, and more hemophiliacs in Japan might show symptoms of AIDS in the future as in the United States.