Epstein-Barr virus-associated early antigen and viral capsid antigen were efficiently induced in human lymphoblastoid [Burkitt''s lymphoma] P3HR-1 cells with culture fluid of F. nucleatum, a member of the indigenous microbial flora of the human host. This finding may suggest a new approach to assess the possible role of the cofactor(s) in the etiology of Epstein-Barr virus-related diseases.