Autoantibodies reacting with the human Ro 60 kDa protein are present in anti-Ro/SS-A positive sera from patients with several different connective tissue diseases including Sjögren's syndrome and systemic lupus erythematosus. To investigate the humoral immune response to this protein, the pattern of antibody recognition of recombinant Ro 60 kDa proteins encoded by both full-length and deletion clones was analysed by immunoblotting. An antigenic region recognized by nearly all anti-Ro 60 kDa positive sera was found to reside in the middle part of the protein. In addition, some sera reacted with two other antigenic regions located in the amino-terminal and carboxyl-terminal part of the protein. For further mapping, overlapping peptides covering the most frequently recognized region of the protein were synthesized by solid-phase methods and used as antigens in ELISA. Three major patterns of reactivity to Ro 60 kDa peptides were found. These results not only indicate the presence of an immunodominant region but also heterogeneity in the autoimmune human response to the Ro 60 kDa antigen.