Sera from approximately two-thirds of patients with rheumatoid arthritis contain an antibody which is reactive with a nuclear antigen present in human B-lymphocyte tissue culture cells. The immunological reaction can be demonstrated by precipitation and immunofluorescence. Evidence is present that the reactive nuclear antigen is associated with Epstein-Barr (EB) virus-transformed lymphocytes. Normal human peripheral blood lymphocytes did not contain the nuclear antigen reactive with rheumatoid arthritis sera, but after infection with EB virus, they showed increasing amounts of reactive nuclear antigen as the cells were transformed into continuous lines. Several established human and simian lymphocyte cell lines known to carry EB viral genomes were shown to contain rheumatoid arthritis-associated nuclear antigen. Evidence is presented which suggests that the rheumatoid arthritis-associated nuclear antigen is different from the previously described EB nuclear antigen.