nRNP Complex antigen is a non-histone nuclear protein composed of several polypeptides of differing molecular weights. It has been shown that RNP and Sm are distinct antigenic sites on the same nuclear molecular matrix consisting of RNA and eight polypeptides, including 70 kD, A(33 kD), C(22 kD) and the Sm peptides B (26 kD), B'(27 kD), D(13 kD), E/F (11 kD doublet), and G (9 kD). Autoantibodies against the nRNP antigen precipitate the U1a and U1b small nuclear RNAs (2). The nRNP antigen is involved in normal post transcriptional, premessenger RNA processing to excise introns. (3) It has been demonstrated that the nRNP antigen is both RNase and tryptic digestion sensitive (4). Autoantibodies to the nRNP antigen, in high titers, have been strongly associated with patients who have Mixed Connective Tissue Disease (MCTD) (5). Other studies have shown that antibodies against the 70 kD polypeptide are found more frequently in sera of MCTD than in anti-nRNP positive SLE patients (6). It has been suggested that patients showing anti-nRNP antibodies tend to have some of the clinical features of MCTD, regardless of their overall disease diagnosis (7). Anti-nRNP antibodies only bind the U1 RNP while the anti-Sm binds with the U1 RNP, U2RNP, U4/6RNP, AND U5RNP. Anti-RNP antibodies are often accompanied by anti-Sm antibodies (1). Double immunodiffusion was the original method for detecting anti-nRNP. Passive hemagglutination was popularized later (8).