A batch of 1025 serum samples, obtained from healthy subjects and hospital patients from five Pacific islands (Viti Levu, Funafuti, Niue, Rarotonga and Upolu) were tested for antibody to hepatitis A virus (anti-HAV), hepatitis B surface antigen (HBsAG) and antibody to hepatitis B surface antigen (anti-HBs) by solid phase radioimmunoassay (SPRIA). High frequencies of anti-HAV were observed in each population (Funafuti, 79.9%; Upolu, 81.6%; Viti Levu, 84.3%; Rarotonga, 95.0%; and Niue, 95.2%), and the age-specific prevalence suggested that the majority of infections with this virus had occurred in the first decade of life. Hepatitis B was endemic in each population, although the total infection rates (as measured by the sum of HBsAG and anti-Hbs frequencies) were lower than for hepatitis A. While infections with each virus tended to run in parallel, peak prevalence of anti-HAV was usually reached in the second decade of life and peak prevalence of HBsAg and anti-HBs was not attained until a decade later. The pattern of infection with HBV was similar in each of the groups studied with the exception of Indians living in Viti Levu, among whom unusually low levels of antigen and antibody were demonstrated.