The Epstein-Barr virus (EBV) genome encodes a protein in its BamHI C restriction fragment rightward open-reading frame-1 (designated BCRF1 or viral interleukin-10 [vIL-10]) that shares protein homology and biologic properties with human IL-10. Several EBV disorders are characterized by prolonged active EBV infection. Because continued EBV replication could allow for increased vIL-10, ELISA and immunoprecipitation were used to determine whether vIL-10 expression during chronic active EBV infection resulted in vIL-10 and IL-10 antibodies. IL-10 antibodies were assayed in patients diagnosed with chronic and acute infectious mononucleosis (CIM, AIM), nasopharyngeal carcinoma (NPC), and EBV-associated lymphoproliferative disease (LPD), as well as from healthy organ transplant patients and EBV-negative or EBV-positive persons. Whether anti-IL-10 antibodies could inhibit IL-10 biologic activity was determined. vIL-10 antibodies were found in CIM, NPC, and LPD patients and antibodies reactive to IL-10 were found in CIM patients. One CIM patient had IL-10 antibodies that neutralized IL-10 bioactivity in vitro.