- The sera of various rheumatic and autoimmune diseases were examined for the presence of anti-RNP/Sm activity. An enzyme-linked immunosorbent assay (ELISA) was employed. Anti-RNP Ab's were detected in 18%, 20%, 28%, 16% of the sera of SLE, myasthenia gravis (MG), rheumatoid arthritis (RA) and thyroid diseases respectively. The anti-RNP Ab's belonged to the IgG and IgM isotypes. Most of the IgG anti-Sm antibodies were detected in SLE sera, but they were found also in two sera of MG and in one sera of RA patients. IgM anti-Sm antibodies were not found in SLE sera, but they were detected in low titer in MG, RA and autoimmune thyroid diseases. The activity against RNP and/or Sm was further confirmed by employing immunoblotting assays. In none of the patients, except those with SLE, was any clinical manifestation of SLE noted. The mere presence of anti-Sm antibodies of the IgG isotypes is not sufficient for the development of SLE, however, its presence is highly specific for SLE.