The Ribosomal P antigen consists of three proteins of the 60S ribosomal subunit (3). These three proteins are designated PO(38 kD), P1(19 kD), and P2(17 kD). Studies have suggested that one P1 homodimer and one P2 homodimer are attached to PO by the NH2-termini (4). P1 and P2 are believed to be the eukaryotic equivalent of the E. coli ribosomal protein L12 and have been shown to contain sequences that are highly conserved among eukaryotes (5). The major immunoreactive epitope of these ribosomal antigens has been mapped and is localized to the carboxy terminus. PO, P1, and P2 have an identical 17 amino acid carboxy terminus. The Ribosomal P antigens have GTPase activity (6) and have been shown to be phosphorylated by casein kinase II (7). Autoantibodies to Ribosomal antigens were originally described inpatients with Systemic Lupus Erythematosus (SLE), and have been associated with psychotic episodes in SLE (1). Anti-P antibodies are detected in the sera of 10-20% of SLE patients and are found in 80% of patients with Lupus Psychosis as opposed to SLE patients with other neurological manifestations (2). The results of one study have indicated that Anti-P antibodies show moderate restriction in spectrotype, isotype, subclass, and light chain type. (8) It has also been shown that Anti-Ribosomal P antisera may contain antibodies to a small fragment of 28SrRNA (6). Another study has shown a high frequency of Anti-Ro/SS-A in patients with Anti-P, while Anti-La/SS-B is present only in a low frequency (1). Other studies have presented evidence that Anti-P antibodies (IgG) may bind to the ribosome in vivo and inhibit protein synthesis (9). The original method for determining antibodies to ribosomal proteins was immunofluorescence.