In this study, compounds with a carboxy ester in lieu of the phosphate ester at the 3'-position have been employed to inhibit the ribonucleolytic activity of ribonuclease A (RNase A). Phosphates at the 3'-position of pyrimidine bases are well-known inhibitors of the protein. We have investigated the inhibition of RNase A by 3'-O-carboxy esters of thymidine. The compounds behave as competitive inhibitors with inhibition constants ranging from 42 to 95 microM. The mode of inhibition has also been confirmed by (1)H NMR studies of the active site histidines of RNase A. Docking studies have further substantiated the experimental results. The compounds are also found to inhibit the ribonucleolytic activity of angiogenin, a homologous protein and potent inducer of blood vessel formation.