Antibodies to rheumatoid arthritis nuclear antigen(RANA) were detected with a double immunodiffusion technique in 0.4% agarose using saline soluble extract from cultured Raji cells as a cellular antigen. Anti RANA antibody was found in 59 of 60(98%) sera from patients with rheumatoid arthritis(RA), 43 of 45(96%) with systemic lupus erythematosus, all 10(100%) with polymyositis or dermatomyositis, all 6(100%) with sclerodermatomyositis, 16 of 17(94%) with scleroderma, 20 of 21(95%) with mixed connective tissue disease, all 32(100%) with undifferentiated connective tissue disease having anti RNP antibody, 44 of 45(98%) with sicca complex and 40 of 44(91%) from normal subjects. There was no significant difference among the prevalence of this antibody. Anti RANA titers were compared between RA patients and normal persons. The geometric mean titer in RA patients was 24.8 and that in normal subjects was 14.6. The former was significantly higher than the latter(P<0.05). The median of the titers, however, was 16 in both groups and the difference of the titers between two groups was not considered to be useful in clinical diagnosis. The correlation coefficient between anti RANA titers and the titers of antibodies to Epstein-Barr nuclear antigen(EBNA) was 0.54(P<0.01) in 44 normal subjects. RANA was detected in extracts from B lymphoblastoid cell lines, Raji and B95-8, and was not present in extracts from B lymphoblastoid cell line, BL, or T lymphoblastoid cell line, MT-1. Raji and B95-8 contained EBNA but BL and MT-1 did not. The present study suggests that anti RANA antibody is closely related to anti EBNA antibody. The presence of anti RANA antibody might suggest prior infection with Epstein-Barr virus.