It was found that the presence of an amino-space modifying thymidine in an oligonucleotide interfered the transfer reaction of [γ- ³²P] ATP bv T₄ polynucleotide kinase.When this modified thymidine located in the middle of oligonucleotide,50% activity of T₄ polynucleotide kinase was inhibited.When this modified thymidine located at the 5'-terminus,only 2% activity remained. Comparatively,those modifications did not affect the activity of T₄ RNA ligase.