Anti-Ro/SSA antibodies are antinuclear antibodies most commonly found in patients with Sjögren's syndrome, a chronic autoimmune disease characterized by dryness of the eyes and mouth. The autoantibodies recognize a RING-finger protein, Ro52/SSA (52 kDa), whose function is still unknown. In this study, the ubiquitination of Ro52 was investigated. We found that Ro52 was strongly conjugated by a single molecule of ubiquitin in cells. Although the biological relevance of this mono-ubiquitination was not defined, the function of Ro52 might be modified by the mono-ubiquitination. We also found that Ro52 was conjugated with poly-ubiquitin chain in cells (poly-ubiquitination), suggesting that Ro52 may be downregulated by the ubiquitin-proteasome pathway in vivo. Interestingly, sera from patients with Sjögren's syndrome showed heterogeneity in their reactivity to poly-ubiquitinated Ro52, probably because of their differing antigenic determinants. This heterogeneity of the reactivity might be associated with the varying clinical features found in patients with Sjögren's syndrome.