Oxidative stress is thought to be involved in the progression of human immunodeficiency virus type 1 (HIV-1)-induced disease. We examined the effect of superoxide (O2-) on HIV-1 spread in cultured human CD4+ cell lines. The O2- significantly enhanced cell-to-cell transmission of HIV-1, although its effect on HIV-1 replication was not evident, presumably due to its cytostatic activity. The effect was notable on the HIV-1 transmission from macrophages to T lymphocytes by endogenous, macrophage-generated O2-. This amplification was specifically reduced to the steady-state level by antioxidants, and further to the basal level by anti-CD4 antibodies, indicating the specificity of O2- for enhancing HIV-1 spread by cell-to-cell transmission.