Autoantibodies may play an important role in the pathogenesis of central nervous system (CNS) disease in systemic lupus erythematosus (SLE). We obtained cerebrospinal fluid (CSF) and, in some cases, sera from 19 SLE patients with CNS lupus and from 12 SLE patients without CNS lupus. Autoantibodies to saline soluble cellular antigens were detected in the CSF of lupus patients and reflected those present in the serum. These antibodies were distinct from the previously described antineuronal antibodies. Analysis of the fine specificities of the anti-saline soluble cellular antigen antibodies revealed that the antiribosomal P protein antibody was present in 4 of 4 patients with lupus psychosis and was enriched in the CSF of 1 patient. Sera containing antiribosomal P protein showed prominent cytoplasmic staining of human cortical neurons, as well as an epithelial cell substrate. These observations, together with the increase in intrathecal IgG synthesis detected in 71% of patients tested, suggest that several populations of antibodies may contribute to the enhanced immunologic activity in the CSF of CNS lupus patients.