Virtually all mRNAs in eucaryotes end in a poly (A) tail. This tail is added posttranscriptionally. In this report, we demonstrate that the enzyme that catalyzes this modification is identical with an activity first identified 30 years ago, the function of which was previously unknown. This enzyme, poly (A) polymerase, lacks any intrinsic specificity for its mRNA substrate but gains specificity by interacting with distinct molecules: a poly (A) polymerase from calf thymus, when combined with specificity factor (s) from cultured human cells, specifically and efficiently polyadenylates only appropriate mRNA substrates. Our results thus demonstrate that this polymerase is responsible for the addition of poly (A) to mRNAs and that its interaction with specificity factors is conserved.