The presence or absence of autoantibodies against citrullinated proteins (ACPAs) distinguishes two main groups of rheumatoid arthritis (RA) patients with different etiology, prognosis, disease severity, and, presumably, disease pathogenesis. The heterogeneous responses of RA patients to various biologics, even among ACPA-positive patients, emphasize the need for further stratification of the patients. We use high-density protein array technology to evaluate ACPA reactivity. Identification of the proteome recognized by ACPAs may be a step to stratify RA patients according to immune reactivity. Methods: Pooled plasma samples from 10 anti-CCP negative and 15 anti-CCP positive RA patients were assessed for ACPA-content using a modified protein microarray containing 1,631 different natively folded proteins citrullinated in situ by protein arginine deiminases (PAD) 2 and PAD4. Results: IgG antibodies from anti-CCP positive RA plasma showed high-intensity binding to 87 proteins citrullinated by PAD2 and 99 proteins citrullinated by PAD4 without binding significantly to the corresponding native proteins. Curiously, the binding of IgG antibodies in anti-CCP-negative plasma was also enhanced by PAD2- and PAD4-mediated citrullination for 29 and 26 proteins, respectively. For only 4 proteins, significantly more ACPA-binding occurred after citrullination with PAD2 compared to citrullination with PAD4, while the opposite was true for 1 protein.