The precise antigenic peptide of Sm antigen (U small nuclear ribonucleoproteins, U snRNP) and the clinical significance of anti-Sm antibodies in systemic lupus erythematosus (SLE) detected by a sensitive radioimmunoassay were studied. By immunoblotting, anti-Sm sera reacted with B'/B peptides of U1 snRNP and D peptide of U1-U2 snRNP, while anti-U1 RNP recognized 68k, A, B'/B and C peptides. Reblotting study using the specific antibodies against each peptide eluted from the blot sheets demonstrated that B'/B and D peptides shared a common Sm antigenic epitope. We purified D peptide from U1 snRNP and established a radioimmunoassay. Anti-Sm antibodies were found in 30% of the patients with SLE using this method, which was more sensitive than the conventional assays. Anti-Sm was associated with late onset renal disease and poor prognosis when analyzed with Cox's regression model.