In 1986, we reported the discovery and isolation of a novel human herpesvirus (HBLV) from AIDS and other lymphoproliferative disorders. Because HBLV is distinct from other members of the herpesvirus family and can infect B- and T-lymphocytes and other human cells (megakaryocytes and glioblastoma cells), we suggested human herpesvirus-6 (HHV-6) as the taxonomic designation for this virus. In cultures from patients' peripheral blood, the evidence of HBLV can be recognized from the appearance of short-lived giant cells (2-10%), which are large, refractile, and are often mono- and binucleated. As these cells degenerate, extracellular virus particles are found in the culture medium. HBLV can infect fresh mononuclear cells, established B- and T-lymphoblastoid cell lines, megakaryocytes and glioblastoma cell lines. HBLV infection can be detected by: a. morphological changes; b. indirect immunofluorescence assay, in situ hybridization, southern blot analysis, polymerase chain reaction amplification; and c. electron microscopy. Because of its wide cell tropism, HBLV DNA sequences have been detected in B-cell lymphomas and short term cultured cells from Sjogren's patients. Expression of HBLV RNA was also detected in sarcoidosis. The etiological role of HBLV in human tumors is unclear. While in vitro data may not necessarily apply to in vivo conditions, the infection of various cell lines from tumors and fresh mononuclear cells suggests HBLV involvement in a variety of diseases.