It has been reported that reactivation of human herpesvirus-6 (HHV-6) causes a failure of hematopoiesis. To clarify the mechanisms of bone marrow suppression induced by HHV-6 infection, it is necessary to establish an in vitro model of HHV-6 infection in hematopoietic progenitor cells. We have established two novel Philadelphia chromosome-positive myeloid cell lines, SAS413 and SAS527, which possess different hematologic characteristics and show distinct susceptibility to infection by HHV-6, from a patient with blast crisis of chronic myelogenous leukemia (CML). HHV-6 subgroup A (HHV-6A) showed marked replication in SAS413, forming syncytia and inducing cell lysis in short-term culture. On the other hand, HHV-6A-inoculated SAS527 continued to proliferate without cell lysis and only a few cells showed HHV-6 antigen expression. In contrast to HHV-6A infection, inoculation with HHV-6 subgroup B (HHV-6B) did not induce any cytopathic effect (CPE) or viral antigen expression in either of the cell lines. Although HHV-6B replication was undetectable, the presence of the HHV-6 genome in both cell lines was shown by polymerase chain reaction (PCR) during culture for more than 10 months, suggesting that HHV-6B latently infected SAS413 and SAS527. Phorbol ester treatment of SAS527 latently infected with HHV-6B resulted in reactivation of HHV-6, as shown by the appearance of a CPE, positive reactivity for the HHV-6 antigen, and isolation of infectious HHV-6. These novel cell lines should be useful for studying the mechanisms of HHV-6-induced hematopoietic failure and HHV-6 latency and reactivation, as well as differentiation, of the myeloid cell lineage.