HIV infection of CD4-bearing lymphocytes alone does not fully explain the immune dysfunction of AIDS. Monocyte/macrophages infected with HIV may serve as a reservoir of HIV, may function abnormally and may transmit infection to other susceptible cells, thus playing a central role in the development of the immunodeficiency of AIDS. Quantitative analysis of surface antigens and measurement of HIV antigens in infected cells have been difficult using the conventional approach of immunofluorescent staining of cells on slides. We have developed a system that maintains monocyte/macrophages in suspension culture for at least four months. Through immunocytofluorographic single cell analysis we have shown that CD4 antigen is present on monocytes, and that a tenfold increase in expression occurs during the first two weeks in culture. In contrast, LeuM3 antigens decreased to background levels in the course of long-term culture. Using anti-HIV p24 antibody, we have demonstrated that monocyte/macrophages can be infected with HIV. Up to 70% of cells from individual donors could be infected. The techniques herein described allow in vitro quantitation of some of the mechanisms by which HIV infection of monocyte/macrophages may contribute to the immunodeficiency states associated with AIDS.