Sera from 141 infants aged 0–12 months were examined for IgG antibodies to HHV-6, HSV, CMV, VZV and EBV and for HHV-6 specific IgM. Following the decline in maternal antibody, antibody to HHV-6 was found to rise by 5–6 months and approached the level found in adults by 11–12 months. In contrast the antibody rates for the other herpesviruses were much slower to rise, especially in the case of CMV and EBV. HHV-6 IgM antibodies were detected mainly in age groups showing a rapid rise in antibody to HHV-6. HHV-6-IgM was not detected in 235 cord blood samples. The data suggest that HHV-6 infection is acquired horizontally, at a very early age in Western Australia.