Various new 5-substituted 3'-azido-and 3'-amino derivatives of 2'-deoxyuridine and 2'- deoxycytidine have been synthesized and biologically evaluated. Among these compounds, 3'-amino-2', 3'-dideoxy-5-fluorouridine(3), 3'-amino-2', 3'-dideoxycytidine (7a), and 3 '-amino- 2 ', 3 '-dideoxy-5-fluorocytidine (7c) were found to be the most active against murine L1210 and sarcoma 180 neoplastic cells in vitro, with an ED5o of 15 and 1 pM, 0.7 and 4 pM, and ...