Cells of two EBNA (Epstein-Barr virus nuclear antigen)-negative human lymphoma cell lines, BJAB and RAMOS, were infected with two strains of Epstein-Barr virus (EBV). In two different experiments, B95-8 virus-infected BJAB cells revealed a gradually increasing number of EBNA-positive cells. Twenty weeks after infection almost 100% of the cell population expressed this antigen. In contrast, it has not so far been possible to convert RAMOS cells into an EBNA-positive cell line. The initial proportion of 35% EBNA-positive cells declined to about 10% 20 weeks after infection. The development of EBNA-positive multinuclear giant cells was a characteristic feature of infection with B95-8 virus. EA (early antigen) and VCA (virus capsid antigen) appeared in less than 0.1% of the cell population after induction with IUdR only. Infection of BJAB and RAMOS cells with P3HR-1 virus finally resulted in both cases in EBNA-positive lines. In contrast to B95-8 virus, the number of EBNA-positive lines. In contrast to B95-8 virus, the number of EBNA-positive cells remained below 1% during the first 6 to 8 weeks. A sudden increase occurred thereafter, bringing the number of EBNA-expressing cells to almost 100% within the following 4 weeks. During this period, BJAB but not RAMOS cells revealed a small number of EA- as well as VCA-positive cells (less than 0.1%). Thus, reinfection by spontaneously released virus may explain the sudden increase in EBNA-positive BJAB cells. Two distinct patterns of EBNA staining in P3HR-1 virus-infected cells were observed. They may suggest a genetic heterogeneity of this virus preparation.