Antibodies to ribosomal protein components were examined in sera from 89 patients with active systemic lupus erythematosus (SLE). The anti-P, anti-S10 and anti-L12 antibodies were detected in 37, 28 and 2 patients, respectively. They were not detected in patients with various autoimmune diseases other than SLE or in healthy donors, which indicates that antiribosomal protein antibodies in sera are specific for patients with SLE. These antibodies are prevalent in patients with SLE and should provide good markers for the diagnosis of this disease.