Severe chronic active Epstein-Barr virus (EBV) infection is a lymphoproliferative disease characterized by extremely high antibody titers to EBV, fever, lymphadenopathy, hepatosplenomegaly, and pancytopenia, without any prior immunological abnormality. A spontaneous lymphoblastoid cell line was established from a 4-year-old boy with severe chronic active EBV infection. Immunofluorescence and Western blotting analyses showed that the cell line was of B cell origin and expressed Epstein-Barr nuclear antigens 1, 2 3a, 3b and 3c, and latent membrane protein 1, which are reported to be targets for EBV-specific cytotoxic T lymphocytes (CTL). The cytotoxicity of peripheral blood mononuclear cells derived from the patient and his HLA-identical sister was assayed against the cell line. The cell line was recognized and killed by anti-EBV CTL derived from the HLA-identical sister, but the patient's peripheral blood mononuclear cells had no cytotoxicity. We conclude that antigen presentation in the EBV-infected cells from the patient is intact and sufficient for generation of an EBV-specific CTL response. These observations suggest that severe chronic active EBV infection may not be caused by impaired EBV-antigen presentation of the infected cells but by impaired cellular immune responses to the virus. Our results also suggest the therapeutic possibility that this disease may be treated by adoptive transfer of EBV-specific CTL or bone marrow transplantation from an HLA-matched donor whose immune response to EBV is intact.