A new polyclonal antibody against the a-polymerase catalytic polypeptide was prepared by using homogeneous HeLa cell a-polymerase. The antibody neutralized a-polymerase activity and was strong and specific for the a-polymerase catalytic polypeptide (Afr 183000) in Western blot analysis of crude extracts of HeLa cells. The antibody was used to screen a cDNA library of newborn rat brain poly(A+) RNA in Agtil. A positive phage was identified and plaque purified. This phage, designated Apolal.2, also was found to be positive with an antibody against Drosophila a-polymerase. The insert in Apolal.2 (1183 base pairs) contained a poly(A) sequence at the 3’ terminus and a short in-phase open reading frame at the 5’ terminus. A synthetic oligopeptide (eight amino acids) corresponding to the open reading frame was used to raise antiserum in rabbits. Antibody affinity purified from this serum was found to be immunoreactive against purified a-polymerase by enzyme-linked immunosorbent assay and was capable of immunopreci-pitating a-polymerase. This indicated the Apolal.2 insert encoded an a-polymerase epitope and suggested that the cDNA corresponded to an a-polymerase mRNA. This was confirmed in hybrid selection experiments using pUC9 containing the cDNA insert and poly(AH-) RNA from newborn rat brain; the insert hybridized to mRNA capable of encoding a-polymerase catalytic polypeptides. Northern blot analysis of rat brain poly(AH-) RNA revealed that this mRNA is ~5.4 kilobases.