The ability of purified Ul small nuclear RNA-protein complexes (Ul snRNPs) to bind in vitro to two RNAs transcribed from recombinant DNA clones by bacteriophage T7 RNA polymerase has been studied. A transcript which contains sequences corresponding to the small intron and flanking exons of the major mouse@-globin gene is bound in marked preference to an RNA devoid of splice site sequences. The site of Ul snRNP binding to the globin RNA has been defined by T, ribonuclease digestion of the RNA41 snRNP complex. A 1517-nucleotide region, including the 5’splice site, remains undigested and complexed with the snRNP such that it can be co-precipitated by antibodies directed against the Ul snRNP. Partial proteinase K digestion of the Ul snRNP abolishes interaction with the globin RNA, indicating that the snRNP proteins contribute significantly to RNA binding. No RNA cleavage, splicing, or recognition of the 3’splice site by Ui snRNPs has been detected. Our results are discussed in terms of the probable role of Ul snRNPs in the messenger RNA splicing of eucaryotic cell nuclei.