To study the effect of human herpesvirus 6 (HHV-6) and human immunodeficiency virus type 1 (HIV-1) Tat in combination on lytic cycle replication of human herpesvirus 8 (HHV-8), we evaluated HHV-8 ORF26 mRNA in BCBL-1 cells fused with HHV-6-infected cells and cocultured with HHV-6-infected cells by direct cell-cell contact or in Transwell system.