2′-Deoxy-4′-thiocytidine (7β), 2′-deoxy-4′-thiouridine (9), and 4′-thiothymidine (10) have been synthesized and evaluated for cytotoxicity in vitro. All these compounds were cytotoxic to L1210, H-Ep-2, and CCRF-CEM cell lines. 4′-Thiothymidine was also active against herpes simplex 1 and human cytomegalovirus in cell culture. © 1991, American Chemical Society. All rights reserved.