The prevalence of hepatitis B viral infection has been evaluated by means of a questionnaire. Contributions were made by 160 institutions from 39 countries and involved more than 400 collaborators. HBsAg was identified by a variety of test kits which were available at the time of the questionnaire. Data are presented for the prevalence of HBsAg in acute viral hepatitis, chronic hepatitis, cirrhosis of the liver and primary liver cancer. Wide variations in antigenaemia were identified in different countries and between the various forms of liver disease. HBsAg is positive more often in chronic hepatitis than cirrhosis. More data using sensitive tests are needed but it appears as if at least one-fifth of the world population has had a previous hepatitis B virus infection.