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Alpha interferon (IFN-a) restricts multiple steps of the human immunodeficiency virus type 1 (HIV-i) life cycle. A well-described effect of IFN-a is in the modulation of viral nucleic acid synthesis. We demonstrate that IFN-α influences HIV-1 DNA synthesis principally by reducing the production of late products of reverse transcription. The magnitude of IFN-α-induced downregulation of HIV-1 DNA and/or progeny virion production was dependent on the IFN-α concentration, the duration of cytokine administration, the multiplicity of infection, the viral strain, and the cycles of viral infection. Interestingly, reductions in viral DNAs could not fully account for the observed IFN-a-induced abrogation of progeny virion production. These data, by our investigation of both single-cycle and spreading viral infections, support a predominant but not exclusive effect of IFN-α on viral DNA synthesis.