The cellular localization of the Ro/SS-A antigen was defined using indirect immunofluorescence and eight monospecific anti- Ro/SS-A antisera which were identified by immunoblotting. Several mammalian tissues were used as substrates. The Ro/SS-A antigen was located mainly in the nucleus of dog liver and Hep-2-cells, and anti-Ro sera had produced a speckled staining pattern. Cytoplasmic fluorescence appeared only in one serum. IgG was the predominant immunoglobulin with anti-Ro/SS-A activity; five sera had complement fixing activity. The sera absorption studies with partially purified Ro/SS-A antigen and the negativation of the ANA tests had confirmed the specificity of our findings.