Antibodies to ribosomal P proteins are markers for systemic lupus erythematosus (SLE) but are often missed by assays utilizing routine immunofluorescence or immunoprecipitation. Expression of autoantigenic sites encoded by complementary DNA (cDNA) clones offers an inexpensive source of antigen for use in quantitative immunoassays. Using a monospecific ribosomal P positive serum to screen a human placental lambda gt11 expression library, a cDNA clone was isolated which reacted with all anti-P human sera tested. Autoantibodies which were affinity purified from the expressed cDNA reacted with the 38 kDa ribosomal Po protein and reactivity was blocked by absorption with recombinant fusion protein. A stable beta-galactosidase fusion protein of 150 kDa was partially purified by a simple differential centrifugation method and used in an enzyme-linked immunoabsorbent assay to reliably quantitate anti-P responses, with a sensitivity and specificity equal to Western blotting. A survey of 84 normals and 151 patients with autoimmune diseases confirmed the high specificity of anti-P antibodies for SLE. The availability of cloned autoantigen will facilitate more detailed clinico-serologic correlations for anti-P antibodies.