3',5'-Diamino-3',5'-dideoxythymidine (7) was synthesized via a nine-step synthesis from thymidine in good overall yield. 3'-Amino-3'-deoxythymidine (8) and 5'-amino-5'-deoxythymidine (12) were prepared with a minor modification of the procedure reported by Horwitz and co-workers. Although the 5'-amino analogue 12 had potent antiviral activity relative to the 3'-amino analogue 8, the latter is a potent inhibitor of the replication of both murine sarcoma 180 cells (ED50 = 5 micrometer) and of murine L1210 cells (ED50 = 1 micrometer) in vitro. Most unexpectedly, however, was the finding of complete lack of either antiviral or antineoplastic activity by the 3',5'-diamino analogue 7 which appears to have acquired the undesirable qualities of both the 3'-amino and 5'-amino analogues of thymidine.