In sera from patients with systemic lupus erythematosus or Sjögren's syndrome, we determined the fraction of antibody that remained reactive with human Ro (SS-A) after absorption with bovine spleen extract, and the reactivity with the 60-kd and 54-kd red blood cell Ro (SS-A) bands by Western blot. Of the 3 groups of sera studied, those containing anti-Ro (SS-A) alone had the highest degree of reactivity with human Ro (SS-A) after absorption with bovine spleen extract, followed, in descending order, by sera containing anti-Ro (SS-A) and anti-La (SS-B), and sera containing anti-Ro (SS-A) and anti-nuclear RNP. The groups of sera could be distinguished on this basis. Sera with anti-Ro (SS-A) and anti-nuclear RNP could also be distinguished from the other 2 types of sera by their uniform and preferential reactivity with the 60-kd red blood cell Ro (SS-A), by Western blot analysis. These findings indicate that there are both qualitative and quantitative differences, associated with the presence of other autoantibodies, in the fine specificity of anti-Ro (SS-A) sera.