Ro, or Sjogren syndrome type A (SS-A), antigen is the most prevalent of the human systemic autoimmune specificities and exists as an inabundant ribonucleoprotein complex (RNP) composed of a 60,649-Da protein, as defined here by cDNA cloning, and the human Y RNAs. The recombinant 60-kDa Ro protein and human Y1 RNA were reconstituted in vitro, and the binding was enhanced by divalent cations. A region of the Ro amino acid sequence revealed a resemblance to the RNP consensus motif found in most RNA-binding proteins. In addition, Ro contained a potential "zinc-binding finger" motif that was distinct from the RNP consensus region and that may participate in the interaction with human Y RNAs or with other proteins. The recombinant Ro fusion protein also proved useful for the detection of autoantibodies in the sera of patients with autoimmune disorders. Possible functions of the Ro RNPs and their relationship to RNA polymerase III transcription are discussed.