Herpesvirus saimiri (HVS) is one of several primate viruses that carry genes for small RNAs. The five H. saimiri-encoded U RNAs (HSURs) are the most abundant viral transcripts expressed in transformed marmoset T lymphocytes. They assemble with host proteins common to spliceosomal small nuclear ribonucleoproteins (snRNPs). HSURs 1, 2, and 5 exhibit sequences at their 5' ends identical to the AUUUA motif, which targets a number of protooncogene, cytokine, and lymphokine mRNAs for rapid degradation. We show that a 32-kDa protein previously demonstrated to bind to the 3' untranslated region of several unstable messages can be UV crosslinked specifically to HSUR 1, 2, and 5 transcripts in vitro, as well as to endogenous HSUR snRNPs. Our results suggest an unusual role for these viral snRNPs: HSURs may function to attenuate the rapid degradation of certain cellular mRNAs, thereby facilitating viral transformation of host T lymphocytes.