THE polysomal mRNA of eukaryotes is associated with at least two tightly binding proteins of approximate molecular weights 75,000 and 60,0001–5. The 75,000 MW polypeptide (P75) is specifically bound to the poly(A) portion of the mRNA2,6,7. Heterogeneous nuclear RNA molecules (HnRNAs), which apparently contain the precursors to the cytoplasmic mRNAs, are also associated with several polypeptides5,8. Although the array of proteins bound to HnRNAs is more complex than that bound to mRNAs, one major protein of MW 75,000 is associated with the poly(A) tracts in the HnRNA8. It has been postulated that the nuclear P75 is identical to the cytoplasmic polypeptide of the same size8. It has also been suggested that P75 is responsible for the transport of poly(A)-containing mRNAs from nucleus to cytoplasm9. However, direct evidence for the function of P75 has been lacking. While characterising poly(A) polymerase (EC 2.7.7.19), the enzyme responsible for the post-transcriptional addition of poly(A) to the 3′ terminus of the mRNA, we observed several similarities with P75. In particular, both occur in the nucleus in soluble and bound states10–12, can be isolated from polysomes2,5,13,14 or post-microsomal cytoplasm13–18, have almost identical amino acid compositions13,19, and have similar MWs of 75,000 and 60,000 (refs 19, 20), respectively. Unfortunately, the conditions required for P75 purification have precluded determination of any associated enzyme activity. We have circumvented this difficulty by raising antibody to poly(A) polymerase, and developing a sensitive and specific radioimmunoassay for this enzyme. We now report that P75 can form complexes with antibodies to poly (A) polymerase and can compete with this protein in the radioimmunoassay. These data suggest that the two polypeptides are structurally similar and, possibly, identical.