CAP structures are added cotranscriptionally to all RNA polymerase II transcripts(1,2). They affect several processes including RNA stability(3-5), pre-messenger RNA splicing(6-11), RNA export from the nucleus(12-14) and translation initiation(15-17). The effect of the cap on translation is mediated by the initiation factor eIF-4F (refs 15-19), whereas the effect on pre-mRNA splicing involves a nuclear complex (CBC) composed of two cap binding proteins, CBP80 and CBP20(11). A role for CBC in the nuclear export of capped RNAs has also been proposed(11,13), We report here the characterization of human and Xenopus CBP20s. Antibodies against recombinant CBP20 prevent interaction of CBC with capped RNAs in vitro. Following microinjection into Xenopus oocytes, the antibodies inhibit both pre-mRNA splicing and export of U small nuclear RNAs to the cytoplasm, These results demonstrate that CBC mediates the effect of the cap structure in U snRNA export, and provide direct evidence for the involvement of a cellular RNA-binding factor in the transport of RNA to the cytoplasm.