Severe combined immunodeficient (SCID) mice reconstituted with lymphocytes from Epstein-Barr virus (EBV) negative human donors develop aggressive tumors after the chimeric mice are infected with EBV. The tumors were composed of human B cells that expressed EBV encoded antigens (latent membrane protein and EBV nuclear antigen2). Southern blot analysis of DNA from 16 SCID/hu tumors with human Ig gene probes showed that each tumor contained multiple heavy and light chain gene rearrangements. Ig kappa gene rearrangements were frequent, while clonal lambda gene rearrangements were infrequent. Analysis of EBV terminal repeat sequences indicated two or more fused termini in each tumor, consistent with a multiclonal origin. Linear terminal repeat segments and viral antigens (EA-D and EA-R) associated with EBV replication were not detected in the tumors. High levels of human Igs in the SCID/hu serum were oligoclonal and primarily contained kappa light chains. Before the appearance of overt tumors, circulating cells with human and EBV DNA could be detected in the SCID/hu mice by the polymerase chain reaction. We conclude that EBV infection in SCID/hu chimeric mice produces a limited number of transformation events, which give rise to oligoclonal tumors resembling EBV-associated lymphoproliferative disorders in some immune-deficient patients.