The stability of the interaction of oligoadenylates, containing single nucleotide substitutions, with cellulose-pdT9 has been studied by thermal elution. In the case of oligodeoxyriboadenylates, the replacement of an internal dA by dC, dG or dT caused destabilization. In the case of oligoriboadenylates, replacement of an internal A residue by C or U resulted in a significantly lesser destabilization. The results indicate that caution should be exercised in extrapolation of data on the specificity of deoxyribonpolynucleotide-deoxyribopolynucleotide hybridization to structure of the type deoxyribopolynucleotide-ribopolynucleotide.