Anti-viral agents are now being considered for the experimental therapy of AIDS, but to date no therapy has been shown to cure HTLV-III/LAV infection or restore the underlying immunodeficiency. The chronicity of infection with HTLV-III/LAV and the propensity of the virus to infect the brain make it necessary to explore new classes of drugs which have the potential for oral administration and penetration across the blood/brain barrier. We tested the capacity of purine and pyrimidine nucleoside derivatives to inhibit the infectivity and cytopathic effect of HTLV-III/LAV in vitro. We focused our efforts on 2, 3-dideoxynucleosides, which as triphosphates are known to be chain terminators of DNA synthesis, and data suggest that the viral DNA polymerase (reverse transcriptase) is more susceptible to the chain-terminating activity than mammalian DNA polymerase alpha. We found that with the ribose moiety of the molecule in a 2, 3-dideoxy-configuration, every purine or pyrimidine suppresses HTLV-III replication in vitro; however, dideoxythymidine had less activity in our system than the others. Substitution of an azido group at the 3-carbon in place of a hydrogen (3-azido-3-deoxythymidine) significantly restored the anti-retroviral effect of the dideoxythymidine derivative. Analysis of five adenosine congeners, differing only in sugar moiety, revealed that reduction (an absence of the hydroxyl groups) at both the 2 and 3-carbons of the ribose was necessary for antiviral effect. Additional reduction at the 5-carbon (the site of phosphorylation after entry into the target cells) nullified the antiviral activity. Studies suggest that nucleosides which are in a 2, 3 dideoxy-configuration may have the capacity to inhibit diverse retroviruses (both human and animal) in vitro. Early trials with 3-azido-3-deoxythymidine have shown good oral bioavailability and penetration across the blood/brain barrier in patients with AIDS.