We tested 989 sera of all age groups (patients and blood donors) from north eastern Germany (West Pomerania and Mecklenburg) and found 820 cases (82.9%) of specific human herpes virus type 6 (HHV 6) antibodies (IgG) gy indirect immunofluorescent assay. The seroprevalence rose to 83.6% when the 7 HHV 6-IgM positive results (0.7%) were included; moreover a further 23 sera (2.3%) showed specific HHV 6 antibodies of both classes (IgM and IgG). The antibody prevalence was constantly high at 90% from the age of 8 months up to the 71-80 years group, and it only decreased in the age group over 80. 93.2% of the newborn infants showed HHV 6-IgG antibodies (of maternal origin) in the cord blood; this prevalence is identical with that for females of reproductive age (92.7% in the 3rd decade, 93.7% in the 4th decade). No sex differences in seroprevalence were observed. The main immunization occurs in the first year of life but infection with HHV 6 at a later age is also well documented. We found the lowest seroprevalence in the 3rd and 4th postnatal months (when maternal immunity had disappeared); later the seroprevalence of specific antibodies rose very rapidly and by the 8th month the rate was the same as for the adult group. The theoretical possibility of prenatal infection due to HHV 6 exists, but the real risk of a first infection during pregnancy is very low, because only 7-8% are susceptible in this group.