Publisher Summary The snRNPs are localized in the nucleoplasm, except for U3 and possibly U8 snRNPs which reside in the nucleolus. Each particle consists of a uridine-rich RNA, which is associated with several proteins. Except for U3, the snRNPs are immunoprecipitated with antibodies from patients suffering from connective tissue diseases. Anti-Sm antibodies recognize proteins that are common to all particles, whereas proteins characteristic for U1 or U2 snRNPs are recognized either by patient sera or by monoclonal antibodies raised against U1 snRNP. High molecular weight RNA elutes from Mono Q at KC1 concentrations above 450 mM and is thus efficiently separated from the snRNPs. Residual tRNA also elutes at high salt concentrations, and only minor amounts remain in the U2 snRNP-enriched fractions. The 7S RNA that coelutes with U2 snRNP from Mono Q is removed by Blue Sepharose chromatography. The U1-specific C protein has not been detected in the stained gel, which may indicate that this protein is underrepresented in the purified U1 snRNPs.