Forty different oligonucleotides were investigated as possible inhibitors of the depolymerizing activity of RNase A. The strongest inhibitors among the diribonucleoside 2'-5' mono- phosphates were: G2'-5'G, C2'-5'G and U2'-5'G, and among the diribonucleoside 3'-5' monophosphates: ApU, ApC and GpU. Of the eight trinucleotides investigated, ApApUp, ApApCp and ApGpUp were the strongest inhibitors. All four dinucleotides studied (ApUp, ApCp, GpUp and GpCp) were very strong inhibitors, ApUp being the strongest one. The results show that the nature of the various bases in the oligonucleotide has an effect on the degree of inhibition, and that the 3' phosphomonoester group increases the binding of the oligonucleotide to RNase A. These inhibitors can be used in physicochemical and biochemical studies of ribonuclease.