Antibody to 2'-5' linked triadenylic acid, A2'p5'A2'p5'A was prepared and purified by affinity chromatography. [8-14C]-A2'p5'A2'p5'A used in immunochemical studies was synthesized by nonenzymatic template-directed condensation. The specificity of the antibody was determined by the inhibition of the binding of the radioisotope probe to the antibody by a competitor. The cross reactivity of a number of modified oligonucleotides has been evaluated in order to gain information on the structural feature involved in the binding to the antibody. The adenine moiety and 2'-5' internucleotide linkage are crucial region for the binding. The cross reactivity of the modified oligonucleotides is indicated to have relation to the biological activity of the corresponding 5'-phosphorylated oligonucleotides.