The Sm proteins B/B', D1, D2, D3, E, F, and G are components of the small nuclear ribonucleoproteins U1, U2, U4/U6, and U5 that are essential for the splicing of pre-mRNAs in eukaryotes. D1 and D3 are among the most common antigens recognized by anti-Sm autoantibodies, an autoantibody population found exclusively in patients afflicted with systemic lupus erythematosus. Here we demonstrate by protein sequencing and mass spectrometry that all arginines in the C-terminal arginine-glycine (RG) dipeptide repeats of the human Sm proteins D1 and D3, isolated from HeLa small nuclear ribonucleoproteins, contain symmetrical dimethylarginines (sDMAs), a posttranslational modification thus far only identified in the myelin basic protein. The further finding that human D1 individually overexpressed in baculovirus-infected insect cells contains asymmetrical dimethylarginines suggests that the symmetrical dimethylation of the RG repeats in D1 and D3 is dependent on the assembly status of D1 and D3. In antibody binding studies, 10 of 11 anti-Sm patient sera tested, as well as the monoclonal antibody Y12, reacted with a chemically synthesized C-terminal peptide of D1 containing sDMA, but not with peptides containing asymmetrically modified or nonmodified arginines. These results thus demonstrate that the sDMA-modified C terminus of D1 forms a major linear epitope for anti-Sm autoantibodies and Y12 and further suggest that posttranslational modifications of Sm proteins play a role in the etiology of systemic lupus erythematosus.