The prevalence of antibody to human parvovirus B19 (anti-B19 IgG) in England and Wales was measured by an antibody-capture radioimmunoassay. Over 2000 sera were examined; 1422 from the general population, 374 from unselected children admitted to hospital and 300 from women attending an antenatal clinic. Waning levels of maternally-derived antibody were found in infants under 1 year old. In children 1-5 years old, 5-15% had anti-B19 IgG and this rose to 50-60% in older children, young adults and women of child-bearing age. In older people, the prevalence of anti-B19 IgG increased with age, rising to more than 85% in those over 70 years old.