The Ro ribonucleoprotein is composed of hY RNA and a 60.7-kD peptide that is antigenic for autoantibodies produced by many patients with systemic lupus erythematosus or Sjögren''s syndrome and mothers of newborns with complete congenital heart block. A major immunoreactive fragment (13 kD) of the 60-kD Ro is bound by 28 of 45 (62%) of the anti-Ro sera tested. Amino acid sequence analysis localizes this fragment to the carboxyl end of the 60-kD Ro peptide. All possible overlapping octapeptides of this 13-kD peptide of 60-kD Ro have been assessed for antigenicity. Sera that bind the 13-kD peptide fragment in immunoblot generally also bind the octapeptides of Ro spanning the sequence AIALREYRKKMDIPA (P<0.01). inhibition="" studies="" with="" synthetic="" peptides="" and="" purified="" ro="" have="" established="" specificity="" for="" reference="" serum="" antibody="" binding="" to="" an="" antigenic="" octapeptide,="" eyrkkmdi,="" from="" this="" region.="" the="" closely="" related="" sequence="" eyrkklmd="" is="" found="" in="" the="" nucleocapsid="" protein="" of="" vesicular="" stomatitis="" virus="" and="" may="" portend="" an="" immunologic="" link="" to="" this="" or="" a="" related="" viral="" antigen.="" these="" results="" also="" demonstrate="" that="" despite="" fine="" specificity="" variation="" between="" human="" sera,="" there="" are="" recurring="" patterns="" of="" anti-ro="" binding="" shared="" by="" some="" patients="" who="" have="" precipitating="" anti-ro="">