2',3'-Dideoxy-3'-aminonucleoside 5'-triphosphates have been shown to be inhibitors of replicative DNA synthesis in isolated nuclei of sea urchin embryo. These compounds inhibit the Okazaki fragment synthesis. The effect of 2',3'-dideoxy-3'-aminothymidine 5'-triphosphate and arabinothymidine 5'-triphosphate is reversible when adding the corresponding substrate for DNA synthesis, 2'-deoxythymidine 5'-triphosphate.