Objective To construct hu-PBL/SCID chimeras and to investigate the development of lymphoma and oncogenicity of the Epstein-Barr virus （EBV）. Methods Human peripheral blood lymphocytes （PBLs） were isolated from healthy adult donors and transplanted intraperitoneally into severe combined immunodeficient（SCID） mice. Mice with hu-PBL engraftment from healthy EBV seronegative donors were injected intraperitoneally with EBV-containing supernatant from suspension culture of B95-8 cell line （active infection）

whereas mice receiving lymphocytes from healthy EBV seropositive donors were not re-infected with B95-8 derived EBV （latent infection）. Pathological examination and molecular analysis were performed on experime-ntal animals and induced neoplasms. Results In the early stage of this experiment

12 mice died of acute graft-versus-host disease

mortality was 34.3% （12/35 mice） with an average life span of 17.5 days. In 19 survival hu-PBL/SCID chimeric recipients from 12 healthy donors

tumor incidence was 84.2% （16/19 mice）. The average survival time of tumor-bearing mice was 65.5 days. EBV-related neoplasms in SCID mice were nodular tumors with aggressive and fatal features. Histological morphology of tumors exhibited diffuse large cell lymphomas. Immunohistochemistry revealed that LCA （CD45） and L26 （CD20） were positive

but both PS1 （CD3） and UCHL-1 （CD45RO） were negative

and EBV products ZEBRA

LMP1

and EBNA2 were expressed in a small number of tumor cells. EB virus particles were seen in the nuclei of some tumor cells by electron microscopy

and EBV DNA could be amplified in the tumor tissues by PCR. In situ hybridization indicated that the nuclei of tumor cells contained human-specific Alu sequence. Conclusions EBV-induced tumors were human B-cell malignant lymphomas. We obtained direct causative evidence dealing with EBV-associated tumor deriving from normal human cells.