Mononuclear leukocytes from human immunodeficiency virus (HIV)-seronegative and -seropositive homosexual men lysed HIV-infected U937 cells to a significantly greater degree than uninfected U937 cells. Depletion of cell subsets with monoclonal antibodies and complement indicated that the effector cells were primarily of the CD16+ phenotype. Acid-stable alpha interferon (IFN-alpha) production induced by the HIV-infected cells correlated with, although was not an absolute requisite for, preferential lysis of the infected targets. The activity of these CD16+, natural killer (NK) cells decreased in relation to the duration of HIV infection and the presence of acquired immunodeficiency syndrome. Pretreatment of peripheral blood mononuclear cells from HIV-seronegative subjects, but not HIV-seropositive men, with IFN-alpha or recombinant interleukin-2 enhanced lysis of both uninfected and HIV-infected U937 cells. These results suggest that IFN-alpha-associated, NK-like mechanisms are active in the cytotoxic response against HIV-infected cells and that HIV infection results in an early and progressive depression of such responses. Prospective investigations may be useful in determining the role of this NK cell response in the natural history and pathogenesis of HIV infection and the efficacy of therapeutic modalities.