Sera from normal subjects were examined for reactivity to human herpesvirus 6 (HHV-6) by the anticomplement immunofluorescence test. Of a total of 179 serum specimens from donors aged from under 10 to 59 years, 141 specimens showed positive reactivity against HHV-6. The positive rate was 70 to 83% for all age groups, and there were no substantial differences in the positive rates. Sera from younger children aged from 0 to 21 months were then examined in detail. The antibody-positive rate of children aged from 0 to 5 months decreased from 52 to 5%, but it gradually increased by 12 months. Almost all children had the antibody against this virus after 13 months of age.