It is important to develop new anti-HIV drugs that are effective against the existing drug-resistant mutants. Because the excision mechanism is an important pathway for resistance to nucleoside analogs, we are preparing analogs that retain a 3'-OH and can be extended after they are incorporated by the viral reverse transcriptase. We show that 4'-C-alkyl-deoxyadenosine (4'-C-alkyl-dA) compounds can be phosphorylated in cultured cells and can inhibit the replication of HIV-1 vectors: 4'-C-methyl- and 4'-C-ethyl-dA show both efficacy and selectivity against HIV-1. The compounds are also effective against viruses that replicate using reverse transcriptases (RTs) that carry nucleoside reverse transcriptase inhibitor resistance mutations, with the exception of the M184V mutant. Analysis of viral DNA synthesis in infected cells showed that viral DNA synthesis is blocked by the incorporation of either 4'-C-methyl- or 4'-C-ethyl-2'-deoxyadenosine. In vitro experiments with purified HIV-1 RT showed that 4'-C-methyl-2'-dATP can compete with dATP and that incorporation of the analog causes pausing in DNA synthesis. The 4'-C-ethyl compound also competes with dATP and shows a differential ability to block DNA synthesis on RNA and DNA templates. Experiments that measure the ability of the compounds to block DNA synthesis in infected cells suggest that this differential block to DNA synthesis also occurs in infected cells.