The Z29 and Ul102 strains of human herpesvirus 6 (HHV-6) were compared for their ability to replicate in fresh peripheral blood lymphocytes (PBL) and in continuous T cell lines. The replication of both strains in PBL was enhanced by mitogenic activation of cell growth. Ul102 replicated in the continuous T cell lines, J JHAN and HSB-2, whereas no Z29 replication was observed in these cell lines as judged by infectious virus yields, the presence of viral antigens, and viral DNA replication. The two strains were compared with respect to their ability to react in immunofluorescence assays with monoclonal antibodies (MAbs) prepared against the GS strain of HHV–6. These MAbs are directed against six different polypeptides including three glycoproteins. All MAbs reacted with cells infected with the Ul102 strain. The Z29-infected cells reacted with four MAbs but failed to react with MAbs specific for an 82- to 105-kDa major surface glycoprotein and with one MAb reactive with a nonglycosylated 180-kDa protein. Taken together, the two strains of HHV-6 exhibit variations with regard to their growth and antigenic properties.