RNA ligase isolated from bacteriophage T4-infected Escherichia coli will utilize a number of different compounds with the general structure Ado-5'PP-X as substrates in an ATP-independent reaction. The P-X portions of these molecules are transferred to the 3'-hydroxyl of an oligoribonucleotide to form a phosphodiester bond, and the Ado-5'P (AMP) portion is released. AMP, CMP, GMP, UMP, dTMP, NMN, alphaNMN, reduced NMN, FMN, Rib-5P, phosphopantetheine, and cyanoethylphosphate all have been added to [Cyd-3H](Ap)3C from their corresponding AMP adducts. Contrary to the relative lack of specificity of RNA ligase for the P-X -group added, the failure of NADP+, deamino-NAD+, epsilonNCD+, epsilon NAD and CoA to react indicates that the enzyme shows a high degree of selectivity for the AMP portion of the substrate. The diversity of chemical groups that can be efficiently added suggests that this reaction of RNA ligase will prove useful for the modification of the 3' ends of RNA molecules.