Epstein-Barr virus (EBV) confers upon normal lymphocytes derived from bone marrow the ability to proliferate indefinitely in a test tube. This process, called immortalization, is crucial to the pathogenesis of EBV infections. Inside the immortalized lymphocyte the EBV genome exists as a complete multicopy circular plasmid which is probably not integrated into the cell chromosome. Most of the viral genetic information is not expressed. However, at least six to eight separate regions of the EBV genome encode viral products which are made in the immortalized cell. The identification of the function of these few genes holds some interesting answers to questions concerning the biochemical mechanisms of control of lymphocyte growth and differentiation.