Sera from 321 children aged 0-179 weeks and 196 adult blood donors were examined for IgG antibodies to human herpesvirus-6 (HHV-6) using an indirect immunofluorescence test. After birth, antibody prevalence declined to a minimum between 20 and 29 weeks. Thereafter the percentage of individuals with antibody increased up to the age of 60-69 weeks after which the prevalence of antibody in the children remained stable at about 88%; in contrast, the seroprevalence in blood donors was 98%, indicating that some individuals remain susceptible to infection after early childhood but that virtually all are infected by the time they reach adulthood. The HHV-6 antibody titre increased steadily over the first 70 weeks of life and then remained stable up to 179 weeks old at a level significantly higher than that of the adults. Two hundred and eighteen of the 321 sera whose HHV-6 antibody titres were 40 or greater were tested for antibody avidity using a modification of the immunofluorescence test whereby low avidity antibody was eluted with urea. The results show that the age distribution of low avidity antibody closely parallels the known distribution of exanthem subitum and, moreover, that the mean antibody avidity increased with time after primary infection. The method was further validated because well-characterised convalescent sera taken from seven children within 3 weeks of exanthem subitum all contained low avidity antibodies. The data presented in this study indicate that low avidity IgG to HHV-6 may be detected after primary infection and that this should prove useful in diagnosis and for seroepidemiological surveys.