Using immunoblotting technique (IBT) antibodies to Sm, RNP and SSB polypeptides we detected in 173 patients with different rheumatic diseases. It was found that antibodies to Sm polypeptides with molecular weights of 28K(B) and 13.5K(D) could be detected almost only in SLE (34.0% and 30.0% respectively). The positive rates of anti-28K and anti-13.5K polypeptides in SLE detected by IBT were significantly higher than those of anti-Sm by counterimmunoelectrophoresis (12.0%) (P less than 0.05). The antibodies to SSB polypeptides with molecular weights of 48K and 43.41K could be detected mainly in Sjogren's syndrome (70.0% and 65.0% respectively) and the antibodies to RNP polypeptides with molecular weights of 68K, 32K(A) and 29K (B') mainly in MCTD (82.6% 100% and 34.8% respectively) (P less than 0.001). It was reported by Pettersson that anti-68K polypeptide had high specificity for the diagnosis of MCTD. But our data showed that the anti-68K polypeptide could also be detected in SLE(26.0%) and PSS(15.0%) and was not so specific for MCTD.