In a study on the prevalence of hepatitis A virus (HAV) exposure among the Basel population in 1978/79, 763 sera matched for age were tested for HAV antibodies by radioimmunassay (HAVAB). Sera of patients with a recent history of liver disease were not included. The results show that the prevalence of HAV antibodies increases with age, a situation similar to that in poliomyelitis before vaccination. The prevalence of HAV antibodies rises from 15% in the 11-20 year age group to 90% in the over-71 age group. The fall in HAV exposure with decreasing age is attributed to a decline in infections due to improvements in hygienic standards during this century, and especially since World War II. The average prevalence of HAV antibodies found in this study is 52%. The results are partly influenced by the high proportion of subjects originating from foreign countries among the population investigated.