-2',3'-Seco nucleosides 5 carrying fluorine and sulfur substituents at C-3' and C-5', respectively, of acyclic sugar moiety were synthesized in enantiomerically and diastereoisomerically pure form. These products and some structurally similar 1',2'-seco-2'-nor-and 1',2'-seco-nucleosides 3 and 4 were tested in vitro for cytotoxicity and antiviral activity. At non-cytotoxic concentrations the compounds were inactive against human immunodeficiency virus and herpes simplex virus type-1.