Abstract: Compositions and methods for detection of anti-citrullinated protein antibodies (ACPAs) in rheumatoid arthritis (RA) patients. Patient samples known or suspected of containing ACPAs were probed against citrullinated proteins, and antibody responses to 190 citrullinated proteins in 20 RA patients were investigated. Unique antibody reactivity patterns in both clinical anti-cyclic citrullinated peptide assay positive (CCP+) and negative (CCP?) RA patients were observed. At individual antigen levels, six novel antibody/antigen complexes were discovered and validated against specific citrullinated antigens (Myelin Basic Protein (MBP), osteopontin (SPP1), flap endonuclease (FEN1), insulin like growth factor binding protein 6 (IGFBP6), insulin like growth factor I (IGF1) and stanniocalcin-2 (STC2)) in RA patients. Identification of immune-dominant epitope(s) for citrullinated MBP was also performed. The identified biomarkers have high specificity, especially MBP.