Sera from some patients with systemic lupus erythematosus and other autoimmune diseases have antibodies against nuclear antigens. An example is anti-Sm sera, which recognize proteins associated with small nuclear RNA molecules [small nuclear ribonucleoprotein (snRNP) particles]. In this paper anti-Sm sera were used to probe immunoblots of various rat tissues. A previously unidentified Mr 28,000 polypeptide was recognized by these anti-Sm sera. This polypeptide, referred to as "N," is expressed in a tissue-specific manner, being most abundant in rat brain, less so in heart, and undetectable in the other tissues examined. Immunoprecipitation experiments using antibodies directed against the cap structure of small nuclear RNAs have demonstrated that N is a snRNP-associated polypeptide. Anti-Sm serum was also used to isolate a partial cDNA clone (lambda rb91) from a rat brain phage lambda gt11 cDNA expression library. On RNA blots, the 450-base-pair cDNA insert of this clone hybridized to a 1600-nucleotide mRNA species with an identical tissue distribution to N, suggesting that lambda rb91 encodes at least part of N. A longer cDNA clone was obtained by rescreening the library with lambda rb91. In vitro transcription and subsequent translation of this subcloned, longer insert (pGMA2) resulted in a protein product with the same electrophoretic and immunological properties as N, confirming that pGMA2 encodes N. The tissue distribution of N and the involvement of snRNP particles in nuclear pre-mRNA processing may imply a role for N in tissue-specific pre-mRNA splicing.