Sm antibodies are a specific serum marker for the diagnosis of systemic lupus erythematosus (SLE), and a high titer of RNP antibodies in the absence of other antinuclear antibodies (ANA) is highly suggestive of the diagnosis of mixed connective tissue disease (MCTD); therefore, specificity is a very important aspect of the assays for these antibodies. Currently in the clinical laboratory, double immunodiffusion (ID) is the most specific of the assays for anti-Sm and RNP. When 74 sera from patients with various systemic rheumatic diseases were assayed by counterimmunoelectrophoresis (CIE) with ID as the reference method, three sera contained nonspecific (non-Sm) precipitin lines in the assay for anti-Sm and three sera contained nonspecific (non-RNP) precipitin lines in the assay for RNP. Precipitin lines formed by the common antinuclear antibodies are more clearly separated by ID than CIE and care must be taken to avoid false identification of precipitin lines with CIE. Indirect immunofluorescence is recommended for ANA screening, ID for definitive antibody identification, and CIE for antibody quantification.