Type I interferon (IFN) inhibits virus replication by activating multiple antiviral mechanisms and pathways. It has long been recognized that alpha interferon (IFN-α) can potently block both early and late stages of HIV-1 replication. The mechanistic basis for the early block(s) to infection is unknown, as is the identity of the participating antiviral factor(s). Here, we define the effect(s) of IFN-α on HIV-1 infection of primary human macrophages and CD4⁺ T cells, as well as several monocytic and T-cell lines. We demonstrate that IFN-α treatment of macrophages, THP-1 cells, and, to a lesser extent, primary CD4⁺ T cells markedly inhibits infection, whereas the effects are minimal in CD4⁺ T-cell lines. Virus entry is essentially unaffected by IFN-α, but substantial decreases (sometimes >99%) in nascent cDNA accumulation correlate closely with losses in infectivity. Interestingly, proteasome inhibitors rescue viral cDNA accumulation, revealing a link between the ubiquitin-proteasome system and IFN-α-induced viral restriction. We also found that diverse primate and nonprimate retroviruses were susceptible to suppression by IFN-α. Importantly, all the primary and immortalized cells used here are proficient at responding to IFN-α, as judged by the induced expression of numerous IFN-stimulated genes, including PKR and OAS1, indicating that a general deficiency in IFN-α responsiveness does not underlie IFN-α's inability to elicit an antiviral state in CD4⁺ T-cell lines. Rather, we speculate that IFN-α fails to induce antiretroviral factors in these cells and that comparative transcriptional profiling with responsive cells, such as macrophages, invokes a strategy for identifying new host-encoded antiviral effectors.