Human herpesvirus type 6 (HHV-6) infection was proved by IgG immunofluorescence method. 354 sera at various age groups were tested and 161 sera from children in more detail. There was no significant differences in our results and obtained by other authors, when testing sera from adults. Our investigation of younger children sera confirmed that mother antibodies are transferred, but also suggest the possibility of virus vertical infection. Our study also confirmed that antibodies are acquired very early in life in the same age group as exanthema subitum does.