A cloned complementary DNA, termed pS2, was isolated from a human fibroblast cDNA library in the bacteriophage expression vector lambda gt11 after screening with a patient's serum containing a high titer of anti-ribonucleoprotein (RNP) antibodies. A reasonable amount of cro-beta-galactosidase fusion protein (pS2EX) was obtained through subcloning of the pS2 insert into a plasmid expression vector pEX-2. Antibody against pS2EX (anti-pS2EX) was purified from this patient's serum by Sepharose 4B conjugated with pS2EX. Immunofluorescent staining of HeLa cells with anti-pS2EX antibody exhibited a typical speckled pattern in the interphase nuclei. In the immunoblot analysis, the anti-pS2EX antibody recognized the 22 kDa protein. Using immunoprecipitation of cell lysate and subsequent RNA analysis, anti-pS2EX antibody was shown to precipitate U1 RNP only. The reactivities of various anti-RNP sera to pS2EX correlated well with the positive reaction to C polypeptide in the immunoblot. These findings indicate that pS2 is a cDNA for C polypeptide of U1 snRNP. In the Northern blot using human RNA and radiolabeled pS2, a single band about 800 base was observed. The nucleotide sequence of pS2 showed no significant homologies to known proteins.