Serum anti-citrullinated BiP (citBiP) antibodies were detected in rheumatoid arthritis (RA) patients at a high frequency, but not in systemic lupus erythematosus (SLE) patients or healthy controls. A. Comparison of the levels of anti-BiP and citBiP antibodies in healthy controls (n = 30), SLE patients (n = 60), and RA patients (n = 100). The serum levels of anti-BiP and citBiP antibodies were measured by ELISA. **: P < 0.01. B. Immunoblotting of the representative serum samples from anti-citBiP antibody-positive RA patients. AMC stands for anti-modified citrulline antibody (Senshu antibody). Band densities were quantified by ImageJ, and the fold increase is indicated. C. The difference between the serum levels of anti-citBiP and anti-BiP antibodies in the serum of RA patients and non-RA controls. ***: P < 0.001. D. Correlation between anti-CCP and anti-BiP/citBiP antibodies. E. Epitope mapping of anti-BiP antibodies. The serum levels of antibodies to native BiP-derived peptides. The sequences of the native BiP-derived peptides are shown in Table 2. The serum samples were taken from anti-BiP antibody-positive RA patients (n = 15, closed circles) and healthy controls (n = 5, open circles). The horizontal lines indicate medians. P-values were calculated by Mann-Whitney U test.