Recent studies have demonstrated that the Ro/SSA autoantigen is heterogeneous as is the corresponding autoimmune response. In addition the autoimmune response is highly species specific and preferentially reactive with the human antigen. Quantitative ELISA study shows that red blood cell Ro/SSA evolves much more rapidly than lymphocyte Ro/SSA and Western Blot analysis shows that the quantitative ELISA results are mirrored by changes in the 60 kD Ro/SSA molecules but not the 52 kD and 54 kD Ro/SSA molecules. The 52 kD and 54 kD Ro/SAA molecules seem to be relatively conserved as indicated by the Western immunoblotting experiments. These studies add weight to the concept that the antigenic epitopes of these related proteins are under the control of separate genes which have undergone different rates of evolution.