Human herpesvirus 6 (HHV-6) induced nuclear antigens in cells as early as 3 h after infection. These nuclear antigens were induced by all three strains of HHV-6 tested, and their de novo synthesis required the function(s) of the intact viral genome. Their appearance was not affected by 2,2'-anhydro(1-beta-D-arabinofuranosyl)cytosine, but was completely inhibited by cycloheximide. However, the nuclear antigens did appear if cycloheximide was replaced with actinomycin D. Thus, the nuclear antigens seem to be equivalent to the immediate early antigens of other herpesviruses.