Patients with systemic lupus erythematosus, with and without nephritis, were studied for their serological profiles. Patients were stratified into two groups defined by the presence or the absence of precipitating antibodies to ribosomal P proteins. It was found that 27 patients were positive for anti-P precipitins and the remaining 34 patients were studied for the presence of lesser amounts of anti-P antibodies by Western blot and ELISA tests. The nephritic patients were characterized most frequently by the presence of both autoantibodies to dsDNA and the ribosomal P proteins. In the group with anti-P precipitins, anti-dsDNA titers were much higher in the patients with nephritis than in those without nephritis. Thus, over a wide range of anti-ribosomal P antibody concentrations, the mutual occurrence of anti-P antibodies with anti-dsDNA antibodies was strongly associated with lupus nephritis.