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Residents of a randomly selected sample of households in a low-income housing project in Bangkok were studied between July 1971 and April 1972. Prevalence of hepatitis B antigen (HBs Ag) in July 1971 was 8.2% in 697 persons age one year and older and changed little with age. Prevalence of antibody to HBs Ag (anti-HBs) was much higher in all age groups and was 46% overall. The prevalence of HBs Ag was higher in males than females from ages 10-29 years and the prevalence of anti-HBs was higher in males up to age 60. The HBV (combined HBs Ag and anti-HBs) age prevalence distribution in July 1971 revealed that over 20% of the children 1-4 years of age had serologic evidence of infection and that the prevalence rose rapidly with age reaching a plateau between 60-70% after age 15. During the nine months of serologic surveillance 13% of individuals acquired either HBs Ag or anti-HBs and 7% lost serologic evidence of HBV infection. The study data suggest that most HBs Ag positive persons are long-term antigen carriers, that their numbers and spatial distribution may be sufficient to account for the wide-spread transmission of HBV in this population, that close person-to-person contact with carriers is probably the most important mode of HBV transmission, and that the susceptibility to becoming an HBs Ag carrier is probably genetically determined.

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