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Individual cells capable of interferon-γ (IFN-γ) synthesis are easily detected by immunofluorescence and flow cytometric analysis using an anti–IFN-γ monoclonal antibody as specific reagent. By IFN-γ flow cytometry assay, we demonstrated that HIV-seropositive patients, starting at the early stage of viral infection, generally have an increased percentage of lymphocytes potentially able to produce IFN-γ, compared with healthy blood donors. IFN-γ expression in patient lymphocytes was observed to increase with the progressive stages of HIV infection, with the highest figures occurring in stage C patients. Such increased IFN-γ expression involved both CD4+ and CD8+ T cell subsets. Most interestingly, we found patients at the same stage of HIV infection who had similar numbers of total and CD4+ lymphocytes but highly different percentages of lymphocytes potentially capable of producing IFN-γ.