Ribosomal P protein is a pentamere composed of 3 phosphoproteins: P0, P1 and P2 with molecular weights, respectively: 38, 19 i 17 kDa. Their function is not exactly known, it is supposed that are essential for all 3 stages of protein synthesis and for regulation of ribosomal GTP activity. Anti-P antibodies react to at least 1 epitope common to all 3 proteins and corresponding to a single sequential antigenic determinant present in the carboxyl-terminal 22-amino-acid sequence. ELISA and immunoblotting are used in detection of anti-P antibodies. There is a wide variation in the reported prevalence of these antibodies in SLE patients. Some authors found it in 5-10% but the others revealed anti-P in 42% of SLE patients. It has been reported that anti-ribosomal P antibodies correlate with high activity SLE, risk of CNS or renal involvement and hepatic or haematologic complications.