Epstein-Barr virus (EBV) displays several biological properties which distinguish it from any other human herpesvirus. The most important of these is that it infects and immortalizes human B lymphocytes, both in vitro and in vivo. These indefinitely growing cells consistently express viral proteins in the nucleus or in the plasma membrane; at least one of these is required for immortalization. In severely immunocompromised hosts such as transplant recipients and AIDS patients, EBV induces B-cell lymphoma. Two strains of EBV (A and B) have recently been recognized; they behave differently in their reaction to EBV-specific cytotoxic T-lymphocytes. Further studies are needed to define more accurately the part played by EBV in Burkitt's lymphoma and nasopharyngeal carcinoma. Besides immortalization, EBV can infect oropharyngeal epithelial cells in vivo, replicating in them at the time of primary infection (infectious mononucleosis) and reactivations. In fact, EBV infects most people throughout the world while rarely causing severe disease.