The cDNA encoding the ribosomal protein P2 antigen was cloned from a human cDNA library constructed in the lambda gt11 expression vector. A beta-galactosidase-P2 fusion protein was purified to near homogeneity and used to develop an ELISA which was highly specific for anti-P antibodies produced in murine and human SLE. The median concentration of human IgG anti-P antibodies in serum was estimated to be 100 micrograms/ml (range 6-450 micrograms/ml). Pre-incubation of human anti-P sera with a synthetic peptide, corresponding to the C-terminal 22 amino acids of P2, completely inhibited reactivity with the fusion protein in the ELISA. These findings confirm that lupus anti-P sera show a striking restriction in epitope specificity and indicate that the P2 fusion protein is a useful alternative to the synthetic peptide antigen for detection and quantification of anti-P antibodies. To investigate the possibility that anti-P antibodies were induced by 'altered-self', cDNA encoding P2 were also cloned from lupus patients and control mononuclear cells. The predicted amino acid sequences of the patients' P2 were identical to that of the normal controls indicating that a primary structural abnormality of the P2 autoantigen was unlikely.