In more than 80% of tumor cells in the pericardiac effusion of a case of malignant B-cell lymphoma, Epstein-Barr virus-determined nuclear antigen (EBNA) was detected by the anticomplement immunofluorescence test. Moreover, herpestype virus particles, although few in number, were demonstrated in the nucleus of lymphoma cells by an electron microscope. Tumor cells in the pericardiac effusion were seeded at 98% purity after centrifugation on Ficoll-Conray and, soon after plating, they proliferated continuously without any lag phase of growth or cell death. Therefore, the established cell line was regarded as of tumor cell origin and named Fujimaki-II cell after patient's name. On the other hand, Fujimaki-I cells were established from the biopsied tumor in the same way. These two cell lines, B-lymphocyte in nature, had both EB virus-related antigens and herpes-types virus particles. Heterotransplantation of cultured cells and tumor tissue obtained at autopsy into athymic nude mice was not successful. Transformation of cord blood lymphocytes by the virus released from Fujimaki-II cell also failed. This might be the first case of non-Burkitt type lymphoma in which the EB virus genome was directly detected in the tumor cells.