In order to study the structure-activity relationships of L-oxathiolanyl nucleosides as potential anti-HIV agents, a series of enantiomerically pure L-oxathiolanyl pyrimidine and purine nucleosides were synthesized and evaluated for anti-HIV-1 activity in human peripheral blood mononuclear (PBM) cells. The key intermediate 8 was synthesized starting from L-gulose via 1,6-thioanhydro-L-gulopyranose. The acetate 8 was condensed with thymine, 5-substituted uracils and cytosines, 6-chloropurine, and 6-chloro-2-fluoropurine to give pyrimidine and purine nucleosides. Upon evaluation of these final nucleosides, the 5-fluorocytosine derivative 51 was found to be the most potent compound among those tested. In the case of 5-substituted cytosine analogues, the antiviral potency was found to be in the following decreasing order: cytosine (beta-isomer) > 5-iodocytosine (beta-isomer) > 5-fluorocytosine (alpha-isomer) > 5-methylcytosine (alpha-isomer) > 5-methylcytosine (beta-isomer) > 5-bromocytosine (beta-isomer) > 5-chlorocytosine (beta-isomer). Among the thymine, uracil, and 5-substituted uracil derivatives, thymine (alpha-isomer) and uracil (beta-isomer) derivatives exhibited moderate anti-HIV activity. In the purine series, the antiviral potency is found to be in the following decreasing order: adenine (beta-isomer) > 6-chloropurine (beta-isomer) > 6-chloropurine (alpha-isomer) > 2-NH2-6-Cl-purine (beta-isomer) > guanine (beta-isomer) > N6-methyladenine (alpha-isomer) > N6-methyladenine (beta-isomer). The cytotoxicity was also determined in human PBM cells as well as Vero cells. None of the synthesized nucleosides was toxic up to 100 microM in PBM cells.