Abstract: In order to explore the causal virus for exanthema subitum(ES),11 patients with ES were studied by virus isolation,electronmicroscopy,serological test and PCR.One virus strain was isolated from seven patients.The isolate could infect human umbilical cord blood mononuclear cels,and the cytopathic effects characterized by large baloon-like cels were observed after infection.Electronmi-croscopy of infected cells in ultrathin sections showed enveloped virus particles which were morphologicaly similar to herpes group virus.Serol ogical test further demonstrated that the isolates were antigenically related to HHV-6GS strain.HHV-6 specific DNA was detected in all the isolates by PCR.All the results confirmed that theisolated virus was HHV-6.In addition,paired sera colected from ES patients during acute and convalescent phases were tested for the antibody against new viralisolates.The antibody titre of convalescent phase was 4 times higher than that of the acutephase,and the convalescent seracould neutralize the infectivity of the new isolates.The results confirmed that the new isolated HHV-6 was the causalagent for ES.

Key words: Exanthema subitum, Herpesvirus hominis