Immunoglobulins, containing antibodies against U1‐snRNP, have been prepared from a patient with systemic lupus erythematosus. After coupling these antibodies to a Sepharose matrix, U‐snRNPs have been isolated and purified from rat liver nuclei by use of immunoaffinity chromatography. The resulting RNPs had the typical protein pattern of U‐snRNPs and a sedimentation coefficient of 12 S. The U‐snRNP preparation was associated with an endoribonuclease which required Mg2+ for optimal activity. The enzyme, with an pH optimum of 6.2, degraded only poly(U). Other single‐stranded polyribo‐ and polydeoxyribonucleotides, tRNA, as well as double‐stranded RNA and DNA were not digested. The products of a terminal digestion are (U)6–12 with 3′‐OH and 5′‐P termini. The possible involvement of this endoribonuclease in the splicing of hnRNA is discussed.