Cyanogen bromide was found to be a highly effective reagent for the coupling of oligoadenylates on a polyuridylic acid template in aqueous solution. Imidazole and divalent metal ions were required for the reaction. Hexaadenylic acid was converted to (pA)12, (pA)18, (pA)24, (pA)30 and (pA)36 in the presence of 0.01 M Co2+ and imidazole. When the reaction was carried out at 25 degrees C for 3 days, the oligomers were obtained in a 34% overall yield.