1. We have compared the sensitivity and specificity of immunofluorescence, counterimmunoelectrophoresis, immunodiffusion, Western blotting and ELISA for the detection of antiribosomal P protein antibodies using 153 lupus sera. 2. Western blotting and ELISA were the 2 most sensitive and specific techniques for the detection of these antibodies. In contrast, cytoplasmic immunofluorescence was observed in only one third of the anti-P-positive patients. Immunodiffusion and counterimmunoelectrophoresis, although highly specific, detecting 14% and 29% of all anti-P-positive sera by Western blotting, were the least sensitive tests. 3. The frequency of anti-P in lupus patients, as detected by Western blotting analysis was 18%. The most frequently observed antibody in anti-P sera was anti-Ro/SSA (39%). Anti-P antibodies were also detected in the sera of 3 patients with negative nuclear immunofluorescence. 4. Anti-P is an additional serological marker for systemic lupus erythematosus and Western blotting is the method of choice for detecting this antibody due to the limited availability of the fusion protein in Brazil.