Carriers of hepatitis B antigen (HB Ag) have been detected in the following groups by immunodiffusion tests: (1) occasional blood donors, of whom 1% were positive; (2) children and adolescents in institutions, with 6.7% positive; (3) people living with HB Ag-positive patients with hepatitis, with 14.4% positive; (4) people living with HB Ag-negative hepatitis patients, with 1.6% positive; (5) people living with patients suffering from other diseases, among whom no carrier was found. Of 114 HB Ag carriers detected in this screening, 18 had a liver biopsy; in all cases minimal hepatic damage was observed. It appears that infection with HB Ag is spread by intimate exposure and that the route may be fecal-oral; it is also likely that the carriers in family groups represent the source of the infection.