Human herpesvirus-6 (HHV-6), a beta-herpesvirus, was discovered in 1986. It is divided into two variants, A and B. HHV-6 variant B is the cause of exanthema subitum, while variant A has not yet been associated to any disease. HHV-6 is ubiquitous and seroepidemiological studies have shown that more than 90% of children older than two years of age are seropositive. HHV-6 infections in the immune competent individual are usually benign, but complications like encephalitis, hepatitis and disseminated disease have been reported. We developed an ELISA for detection of HHV-6 IgG antibodies for seroepidemiological studies and compared it to an immunofluorescence assay (IFA). Both methods revealed a seropositivity of > 90%, and the ELISA was a reliable alternative for measurement of HHV-6 IgG antibodies. The prevalence of variants A and B is not known, since available serological assays do not discriminate between the two variants. Polymerase chain reaction (PCR) and T-lymphocyte proliferation assays were used to identify the distribution of variants A and B in a group of healthy adults. HHV-6 DNA was detected in 22% of the leukocyte samples. All samples that could be typed were variant B. Furthermore, 58% of healthy adults responded to variant B in the lymphocyte proliferation assay and 25% to variant A, but the responses did not correlate to the PCR assays. In order to investigate possible congenital HHV-6 infection we analysed the presence of HHV-6 DNA in matemal blood and in cord blood from 107 mother-infant pairs. In a second group, sequential samples were taken from women at months 3, 5 and 6-8 during pregnancy, and from mother and infant at delivery. Cord blood samples from 104 mother-infant pairs were also analysed. HHV-6 DNA was detected in around 40% of samples during month 3-8 of pregnancy, compared to 25% in non-pregnant age matched controls. HHV-6 IgG antibodies were found in 96%, and the titres were significantly higher in women with HHV-6 DNA in their leukocytes. In 2/211 mother /cord blood pairs we ...