Both human (hu) and viral (v) interleukin-10 (IL-10) appear to be important cofactors in the survival and growth of lymphoblastoid cell lines infected with Epstein-Barr virus (EBV). When mice with severe combined immune deficiency (SCID) are injected with human peripheral blood lymphocytes (PBL) from normal individuals who are seropositive for EBV, the majority of hu-PBL-SCID mice will develop an EBV-associated lymphoproliferative disease (EBV-LPD) of human B-cell origin, not unlike some cases of EBV-LPD that are seen in immunocompromised individuals. The role of huIL-10 or vIL-10 in this chimeric mouse model of EBV-LPD is unknown. In the present study, we show that hu-PBL-SCID mice that develop EBV-LPD have significant elevation of serum huIL-10 levels compared with mice that do not develop EBV-LPD (P = .005). vIL-10 was undetectable in all animals. The EBV+ tumor samples express transcript for huIL-10 and huIL-10 receptor, express huIL-10 protein by immunohistochemical staining, and show specific binding of recombinant (r) huIL-10. In vitro analysis of the functional consequences of rhuIL-10 binding to IL-10 receptors on fresh EBV+ tumor cells shows that rhuIL-10 can prevent programmed cell death as well as promote proliferation and can do so at concentrations of huIL-10 found in vivo. Thus, huIL-10 production by EBV+ tumor cells may contribute directly to their malignant outgrowth in the hu-PBL-SCID mouse by two autocrine mechanisms: prevention of programmed cell death and proliferation. The implications of such findings with regard to EBV-LPD in humans is discussed.