Epstein-Barr virus (EBV) infection is associated with Burkitt's lymphoma (BL) in normal individuals and immunoblastic B cell lymphomas in immunosuppressed or HIV-infected individuals. SCID mice reconstituted with human peripheral blood leukocytes (hu-PBL-SCID) from EBV-seropositive donors also may develop spontaneous B cell lymphomas which histologically and phenotypically resemble post-transplant tumors, and are distinct from BL. These tumors always contain EBV DNA. We have noted three different reproducible outcomes depending upon the EBV-seropositive donor used for generation of hu-PBL-SCID mice: (i) no tumors appear; (ii) tumors appear in a fraction of hu-PBL-SCID mice with a 10-20 wk. latent period; or (iii) tumors appear in all hu-PBL-SCID mice within 6-10 wk. Southern blot analysis of late versus early tumors using a probe specific for the EBV terminal repeat sequences (BamNJ), which allows distinction between circular latent and linear replicating genomes, shows that late tumors do not involve active EBV replication but that early tumors do show replicating genomes. In addition, EBV genomes were monoclonal in late tumors but polyclonal in early tumors. These data suggest two mechanisms for EBV lymphomagenesis, slow outgrowth of rare latently-infected B cells, and more rapid transformation of uninfected bystander B cells by replicating virus. The latter process may be highly amenable to therapy in patients at risk for EBV-related lymphomas. In addition, prospective screening of EBV-seropositive transplant recipients in the hu-PBL-SCID model may predict the risk of post-transplant lymphoma development.